depends where you are going as the counrty differs dramatically - I stayed in Rovinj last May - beautiful countryside (grounds to the hotel) and a 10 minute walk to the caost - a fishing bay, the most gorgous people, food and place. Not much to do but relax! Very few beaches its more schell type. I've heard July onward very very hot, may sept cooler. Hope it helps.
Christian
should still be warm and sunny though you can never be sure anywhere these days. Beaches are only sand in the far south; pebble and shell elsewhere. Istria (the peninsula in the north) is quite green, the coast further south drier and rocky. Dubrovnik is gorgeous (and 90 degrees this week). Islands are worth a visit if you have time.