Split is stunning :) awesome people, beautiful islands - you can take a day trip to Vis, Hvar etc ... but these can be pricey. We took a cat out for 46 euro per person - all drinks plus lunch which stopped off the coast of Brac and Solta ... was lovely! It's called Summer Blues ... nice music we danced some bachata and merengue but they played a mix of party/dance songs too :)
Rovinj is beautiful - Venetian influence is strong :) ... if you go, stop off in Pula at the Roman amphitheatre it's pretty cool :) Rovinj is very calm and chilled.
Otherwise do the cable car in Dubrovnik ... city walls (don't do the entire thing and don't do it at midday it's WARM!) ... and canoeing if you like sporty stuff.
All the food in Croatia is pretty much average (I am South African so I'm used to good food and my expectations are usually too high with food) ... but you can get a toasted sarmie for cheap. There's a nice Mexican place in Rovinj and another place in Split with good food ... can't remember offhand but will try and post it :)
Enjoy the planning :) about 5 years ago