It's difficult to know where to start, but these are my top 3 places in Croatia - posted online in the hope that others find this useful.
They are in no particular order, but I'd strongly advise you to visit them all!
I was only in the area for a week, and these are all easily accessible from Split, which was my nominal base for the week (I moved around a fair bit but flew in and out of Split International airport).
This is a beautiful little island with everything you would expect for a tine place tht has been inhabited since Roman time - cute little squares, tiny walkways through castle-like walls. It has fantastic views (especially from up high, if you make the slog up the hill to the fort), with 360 degrees of sea.
The best bit about Hvar is the food though - with tens of hte very best restaurants in the Med clammering your your attention (and your wallet!). It's not a cheap place to eat, but I would definitley recommend Agava, amongst others. It is THE standout restaurant.
I loved the feel of Trogir. There is an old town that is literally on its own island (and a UNESCO-heritage site), brilliant beaches, and a nightlife that keeps you up until way past your bedtime - especially if you're a fan of euro-trance like I am!
I've seen lots of waterfalls in my time, but with these tehre is so much going on - it's basically 7 different waterfalls piled on top of each other. The approach by boat from Skradin is also nice, as is the fact that instead of piling to see them all and pushing through the crowd, you can just head up to them on your boat. :)