Plitvice was definitely the highlight of my trip to Croatia. Split was nice if you want to go through the markets, assemble lunch, and then dangle your legs off the boardwalk and eat. Direct bus from Plitvice to Split, then you can take the overnight train back to Zagreb.

Another option is to go into Slovenia. Ljubljana is an extremely pleasant city and the nearby caves are really neat. 90-minute train in from Zagreb.