We visited the Dalmatian coast in summer ’06 and liked it quite a bit.
In terms of terrain, it reminded us a lot of Corfu, which makes sense, the two are so close. You could also travel the coast of Bosnia-Herzegovina.
Do a search and look at the photo galleries, see what others have posted.

Dubrovnik is unreal, really nice place with some good stuff to do and its still cheap.  Not the biggest party town at least when we were there, but amazing nonetheless.  There are also alot of cheap and really cool guest houses you can get.  We stayed in one directly on the ocean with its own patio and it was no more expensive than any hostel.  My next trip will be heavy on the Croatia for sure.
 
Head a little south of Dubro into Montenegro if you want something different too.

The Coastal town of Split is gorgeous.

How long would you recommend someone spend in Croatia to really get the whole experience?

also, how difficult was it on the trains to get to and from Croatia? So far it seems near impossible. I just want one Eurail Pass, not 4 or 5…

At least a week.
We just saw the coast over 4 days.

In terms of getting there, we flew from Germany so I don’t know about trains.

I actually wasn’t a big fan of Split (but it wasn’t bad either). The coast is very nice though, as is inland. For instance, Plitvice national park is amazing – do a web search. As for getting there, it’s very easy. I don’t know if they’re on the Eurail pass system, but that’s not always such a bad thing. Unless they’ve radically updated the train coverage in the past few years though, you will find yourself taking buses as well. Not to worry, they are convenient and cheap enough.