If you go and explore some back streets you can find some awesome, tasty and much cheaper restaurants. It is rare to find awful food in Zadar, especially if you like sea food. Go and explore, find something in a side street and you'll have a great experience (and it'll probably save a lot of kuna).
Consider a day trip down to Split. It takes a few hours on a coach, but Split has some really great nightlife.
Doing boat day trips is highly recommended. But make sure you do it when there is nice weather - you don't want to book a trip then on the day find out it is raining hard all day!
The sunset in Zadar has been rated multiple times as the best one in the world.
Even Alfred Hitchcock said "The sunset of Zadar is the world's most beautiful and incomparably better than in Key West, Florida".