Hey Suze, how's it going?
Sounds like you're doing the same trip as we did in September. We went Split-Makarska-Trstenik-Dubrovnik-Mljet-Korcula-Hvar-Split and had an awesome time!
I'm work at the moment, so this is gonna have to be quick, but the main things that stand out are...
Split: The beach at Bacvice (spelling?) and also an open air nightclub on a marina near the football stadium (not sure of the name, but it kicked off on the Friday night before the boat trip!).
Makarska: Bar/club in a cave about 10 mins walk from where the boats moor.
Trstenik: You might not go here (I don't think we were meant to), but we just chilled out on the beach with a few drinks (not much else to do to be honest, but a cool place to just hang out).
Dubrovnik: The walk around the wall of the old town costs about 5 quid and can take a couple of hours if you stop off at a few cafes etc.
Mljet: Definitely go for a swim in the lakes!
Korcula: We weren't there for that long, I think we just out for dinner and to a few bars.
Hvar: We'd missed the main partying season, but it was still a good craic. The boat moored at Stari Grad (about 20 mins by bus from Hvar) and there were a few good spots to go swimming there too.
I hope this helps a bit, enjoy your trip!
Cheers,
Adam