Croatia

Split

Split Riva: basically the waterfront corniche and the main spot to go for a walk, stop at a cafe, and enjoy Split's old town area.
Diocletian's Palace: Main tourist attraction in Split. It's the palace of a Roman emperor.
Klis Fortress: Scenic spot over Split that is a short drive away from the old town.
Stella Croatica - Olive Museum & Traditional food: Next to Klis Fortress, combine with a visit to the fortress. Ideally call them early and ask for veal peka (veal ispod sac) which takes 2-3 hours to cook.
Brac island: The golden horn beach in Bol is the main attraction, but takes some time to get to (ferry plus taxi). A hike up the mountain Vidova will burn lots of calories.

Zadar or Šibenik: both have lots to do and are a bit less touristy than Split. Can choose which one is most interesting to see
Konoba Mondo: In Motovun, the place famous for truffles. Good truffles in large servings. Near Slovenia border
Pizzeria Rumore: Good pizza, between Rijeka and Pula

Željava air base: abandoned Yugoslav air base built into the mountain. Common destination for urban exploration.
Plitvice Lakes National Park: One of the standard tourist places in Croatia. Beautiful nature with lakes and waterfalls
Krka National Park: The one standard tourist place in Croatia. Waterfalls and rivers, nice nature.
Tesla Museum: Tesla appears on both Croatian and Serbia currencies and has a museum in each country, so don't debate whether he is Serbian or Croatian unless you want to start a new Balkan war :)
Sinkhole lake: Red Lake and Modro Lake
Fancy restaurant options include Konoba Campanelo and Pelegrini (reservation recommended)