What do I do and where do I go during a 16-hour stopover in Bangkok?

16 hours is nothing.
Go to a coffee-shop with your laptop. Then walk around. Eat. Take pictures. Listen to the music. Go to coffee-shop again.

Leave your luggage in a locker at the airport.

Take the skytrain to its terminal.

Right there you have the MBK mall on the south. A big mall on 6 stories full of small shops. It is a mix between traditional Thai street markets and modern malls.

Then on the north side of the sytrain terminal there is Siam Paragon, the luxury mall with all luxury brands. This place is beautiful. One hour to visit both malls is enough for me.

Then walk to the north and take the boat bus (20 THB) to the center.

Visit some temple, enjoy street food, get a 2-hour Thai massage or a more relaxing oil massage then head back to the sky train terminal.

Use motorbikes taxi to move, never tuk tuk (noisy, smelly, slow as car, expensive). Always take the smallest portion of street food. You will always see another one you want to try, and if you’re full you can’t enjoy.

Enjoy!

Looking at the image in another answer, follow the white transfer sign, which essentially means transfer to domestic flights. This will lead you to border control, because you’ll be entering Thailand in Bangkok, as you’re continuing domestically (domestic flights are not subject to border control, which is why it’s cleared in Bangkok).

After immigration, follow signs for the gate.

Alternatively, if wishing to visit Bangkok, follow the signs for the exit – however, if your baggage is checked through, it may be a hassle to retrieve it spontaneously.

There two general variants on the stopover:

• Connection without immigration: Domestic to domestic connections, many international to international (although rarely the US) and domestic to international (unless the country has exit immigration). Also international flights within the Schengen zone.
• Connection through immigration: This includes almost all international to domestic connections (some countries have immigration outside of their own country such as US immigration in Canada and Bahamas)

Your case is of the second type since Bangkok will be your port-of-entry to Thailand.

The process is generally to disembark and follow the signs which apply to you. For most people these will be the same up to immigration:

1. Follow the signs to immigration if you come from an applicable flight. Since this is usually required of all passengers, there will usually be no other option, meaning all other hallways and doors will be closed to you. There are generally staff on hand to ensure this and to assist minors, elderly and rare cases what are exempt. When in doubt, you can ask them what to do.

2. Once passed immigration you may have to proceed to baggage claim if your luggage was not checked through to your final destination. They should have told you this at the check-in counter and this is the norm rather than exception now, as long as you have booked the trip as a single ticket. Even, if not, they sometimes can check it through if you ask.

   • In most airports, claiming luggage this will force you to leave the secure area. In this case you follow the signs to the right carousel or drop-off-location of luggage for your flight. When you have collected your luggage proceed to the luggage drop-off area. Some airports have one or more for all flights, some require you to drop it off at the check-in counter for your airline. If you do not see signs for baggage drop, go to the airline counter.
   • Since you left the secure area, you must go through security again. Now is a good time to check your watch and see how much time you have left to decide if there is anything you have time to do before your next flight. More on this further down.

3. Look for Departure Screens scattered throughout the airport. There search for your airline and flight number. Often they are sorted by the destination city. When you find the row for your flight, read it and identify the gate and terminal your next flight will depart from.

4. Follow the sings to the departure gate for your flight. If this is in a different terminal than you are, change terminals first, since gate numbers may repeat between terminals. There will be signs to each terminal. Once in the right terminal, proceed to the gate. You usually must be at the gate 60 to 30 minutes before departure, so make sure you go on time.

Now if you have extra time. Use the facilities. Bathrooms in the airport are much better, spacious and cleaner than those on the plane. Get yourself some snacks if you do not fancy the airplane food and its going to be a long flight. Probably this will not be the case for you going to your destination but on your way back it probably will.

There are variants all over the world on the above but it generally goes like that. Airports are rather serious places, so there are plenty of safety for people not to do the wrong thing although it is your responsibility to reach the departure gate on time.

With a long stopover, there are many things you can do, depending on where you are and the location of the airport. Ask about transport time to wherever you are headed and back to make sure you really have time. Some airports are 1h+ outside any major city, so that is easily 2 hours less.

Things to do on a layover

• Sleep if you are tired. If you just came off from a redeye or are otherwise too tired for anything, find a place to sleep. Your true destination awaits. Many international airports have hotels right in the airport and there are options with free shuttles to transport people back and forth to nearby hotels. Usually the hotel at the airport is expensive but you save a lot of time, so get to sleep more and do not have to worry about return traffic.
• Visiting is an interesting possibility if you are somewhere new. You can get onto one of those tour buses that leave the airport every hour or so and do a quick loop if time is limited and you don’t want to risk unexpected issues. Otherwise, ask at the Tourist Information Counter and find your own way. That can be fun but give plenty of time. I tried it this year in Lisbon and got back to my gate only 10 mins before the flight because 2 trams broke down while I was on my way back!
• Shopping. Again, it depends on the location but some famously cheap places often have shopping shuttles that take people from the airport to a mall or outlet center and back. Never tried this myself but I know people who just do that when they travel!

If you’re going to Koh Samui via Bangkok, you will change planes, indeed, and probably airlines: few airlines go to Samui (it used to be a Bangkok Airways monopoly, and PG still have the lion’s share of flights). I’m also assuming you’re arriving in Suvarnabhumi (since DMK also serves as an international airport for low-cost carriers, there’s a small possibility that you don’t arrive at BKK).

When you arrive in Bangkok you have two choices: enter Bangkok proper, and visit (16 hours is a long time to wait in an airport), or indeed go to the domestic terminal (it’s all the way at one end of the airport), pass through immigration and security, and wait a long time for your plane.

The second choice would be stupid, even if you’re on a budget — nothing is cheap in Suvarnabhumi, and you’ll be bored senseless by the time you board your next plane.

Here is the airport map: the Domestic area is all the way to the left. Immigration in the middle.

Anyway I’d recommend you go to Bangkok and visit. Follow the crowd off the plane (most of them will be indeed heading to Immigration), and keep an eye for the blue signs.

Note that in many cases your luggage can be checked-in all the way to Samui (Bangkok Airways has many interline agreements), but this requires that you stay airside. Not ideal. If you wish to visit Bangkok, you’ll have to pick up your luggage and check it in again later.

If you have a day-time layover, and don’t want to pay for a hotel, you can store your luggage at the coin locker in Phaya Thai station, at the intersection of the Airport RailLink and BTS lines:

The Airport RailLink is the best and cheapest way to go downtown. 45 Baht, and zero traffic. The station is two floors below the arrival level. You go all the way to the terminus, Phaya Thai, and you’re downtown. Plenty of things to do. Since you have a 16-hour layover, it means you really have about 12 hours: 1 hour to pass through Immigration, pick up your bags, find your way to the train. 45 minutes to Phaya Thai. And back. Arrive 1.5 hours before departure: that’s 4 hours taken out of your 16. So you still have half a day in Bangkok. Not bad. Enjoy!

Following the other people on your plane is not the best solution since they could have a connection to a flight different from yours.

There isn’t a general solution to stopover. Sometimes if you book a single ticket for your journey you can get a night in a hotel close to airport. In some other cases you can visit the town or sleep in the airport.

(edit: In case you wish to exit the airport you need to be sure that you have all the documents needed to enter that country)

It’s completely up to you.

Anyway you will not have any issue in the airport since you will find plenty of screens where it’s written time and gate for your next flight.
If signs are not clear you can ask to airport staff and they will help you.