For Dubrovnik, if you are going in the summer, Lokrum Island is pretty nice, but only do that if you have an overnight there.  If not, tour the old city and around it.  If you can, walk the walls.  For Kotor,  we used Mont Travelers and designed a tour of Old Town of Budva, Kotor, and the Lipa Caves.  It was nice, but if you've been to caves before, you might want to focus on another area.  We've been to Venice a couple of times and Athens and do the standard in both.  Love the Plaka as a lunch