I'm going to be around Venice/Padua at the start of September, and will have ~4 days extra to do a bit of exploring with my Euro friend before I return to the US. We want to try something new, meaning dipping into either Slovenia or Croatia. We'd like to enjoy the summer weather and explore by bicycle, so I don't think we'd want to cruise all the way down as far as Split, for example. We're able to go up to ~80 km / day if we need to cover that much ground.
Which country would be more fun to check out, or does anyone have recommendations of things to (not) do? We're down with beaches, happy to experience local culture and cuisine, neat architecture, vistas or scenic areas, pretty much anything apart from clubbing. Random tips or cultural pointers also appreciated.