score:4
Have a look at http://www.autobusni-kolodvor.com/en/terminal.aspx. There's a bus at 7 am that goes to Plitvice (en route to Serbia). It seems like the last bus to Split leaves at 3:45 pm, which gives you just over 5 hours at the park. You need to be at the bus stop early to flag the bus when it comes.
If you get off at entrance 2 at Plitvice (which you can if you take the 7 am bus), you can leave your bag at the park's hotels for a small fee. I'm not sure about entrance 1 but I imagine it would be the same. I would recommend entrance 2 anyway, as most of the morning tour buses enter from entrance 1.
By the way, when I took the 7 am bus last April, Plitvice wasn't an official stop, and according to the bus station's board there were no morning departures to Plitvice. In fact, even the Zadar tourist office tried to convince me that there were no morning departures. If the situation hasn't changed, you certainly can't book a ticket in advance - you have to let the driver know you want to be dropped off at Plitvice and pay on the bus. The bus will almost certainly not be full: I was the one on board when we left Zadar.
Upvote:3
You can leave your bag at the entrance for a small price, I think it was around 2-3 euro. There are some lockers, though the big ones are rare, so in case you have a full-size luggage, you need a bit of luck, but it worked for me.
Going early to the bus stop is very crucial. I didn't reserve any tickets, I don't remember whether it was even possible to do so, and to make sure that I'll catch the bus to Split, I went to the bus stop about 20 minutes before the supposed arrival time of the bus. 10 minutes later there appeared another bus going to Split, which was not mentioned in the schedule, and that one followed a much faster route (which, again, was not mentioned, it was a nice surprise). All in all, that unexpected bus was about 1 euro more expensive than the regular bus I was supposed to take.
Plitvice is definitely worth the trip, it's one of the most beautiful places I've seen in Europe. Enjoy!
TL;DR: Go to the bus stop as early as possible, there might be additional buses or alternative means of transportation, catch the first one that suits you.