Dual US/NL citizen traveling to Edinburgh and Copenhagen

Travel Neither Denmark nor the United Kingdom require visas for American citizens.

Airfares are often very inexpensive in domestic Europe, so your best bet would be to purchase a round trip airfare for BOS-EDI (or CPH) and then round trip tickets for the EDI-CPH (or vice versa). In my personal experience, I was able to secure a round trip ticket from Dublin to London for 100 USD. I would explore different websites. If all you need is a seat then you should be able to find something relatively inexpensive. As for the trip from the US to Europe, any international airline should be fine. I know Norwegian Airways just opened flights from Boston to Copenhagen and you could secure a round trip ticket for less than 500 USD.

Clothing and Footwear It will be cool/cold in both areas during March. Also count on rain. I would recommend something that is waterproof, and will keep you warm. Dressing in layers will be your best bet. Your list of clothing seems sufficient, but it never hurts to pack some extra undergarments.

Safe travels!

Ferry: There are no scheduled ferry crossings between the UK and Denmark anymore. The closest would be Newcastle to IJmuiden, and then ground transport through the Netherlands and Germany. This will be significantly more expensive than a cheap one-way air ticket from Edinburgh to Copenhagen.

Air: For the transatlantic legs, you should look into searching for an "open-jaw" round-trip BOS-EDI + CPH-BOS. Traditional airlines will often be willing to sell this for about the same price as an actual back-and-forth round trip, but you will need to use the "advanced" or "multi-city" mode on their booking forms.

(It will probably not entail any savings to book the EDI-CPH leg as part of this multi-city itinerary, compared to as a separate ticket possibly on a different carrier, but it won't hurt to check).

Passport: If at all in doubt, have both passports ready for presentation if trouble arises. None of the countries involved will give you any trouble for having two passports.

In general you'll want to use the Dutch passport for entry/exit passport controls in any EU country (which will be much smoother than the US one, even though you could use that too), but the US one at check-in for the flight back to US (so the airline won't think you need an ESTA) and for immigration back in Boston.

Climate: I have lived in both Copenhagen and Edinburgh, and their climates are very similar. Both temperatures and rain/snowfall in March should be within the range of variations you would expect in Boston for the same month.

Note: the UK is planning to leave the EU, at the time of writing it has not actually left yet. What impact this will have on travellers going forward is unknown. It is highly unlikely (but not impossible) that this will affect the OPs travel in march but far more likely that it will affect future readers of this answer.

I have been looking at flights from Airlines like Wow, RyanAir, and British Airways. Is there an airline that is best for BOS->EDI? One way prices seem to be around USD$50 less than two way prices BOS<->EDI, which is crazy to me.

You want to get the two transatlantic legs on the same ticket, that way they can be priced at "half-return" rates rather than "single" rates.

There are various ways this could be done from a ticketing perspective.

1. An "open jaw" ticket that covers the two transatlantic links and nothing else.
2. A return from Boston to Edinburgh with a stopover in copenhagen on the way back.
3. A return from Bosten to Copenhagen with a stopover in Edinburgh on the way out.
4. A "multi city" ticket with the thee legs priced seperately.

I would suggest plugging your requirements into the "round trip/multi-city" options on something like google flights and seeing what comes back. I would probablly try this both with just the transatlantic legs and with all three legs.

Should I use my American Passport or my Dutch one, or perhaps some combination of both?

By law you have to use your US passport to Enter the US.

For travel to the UK and Denmark you can use either passport but using the Dutch one puts you in a much stronger legal position. EU citizens have the right to enter EU countries while for US citizens entry is subject to the immigration officer beliving you are a legitimate visitor.

So you should show US border officials your US passport and EU border officials your Dutch passport. Airline officials should be shown the passport for the country you are travelling to.