I've been reading this forum, and thank everyone for the great information. My husband and I have been going around and around planning a September trip (this year). He wants to make it a month long trip, and I think this might be a week too much. We like to drive, use a city guide in various places but also have time to be on our own to enjoy the city or area, have a coffee or wine and just hang out; we'd maybe get a driver into Montenegro or the Falls. We are two active and fit "older" people who enjoy hiking, walking, swimming, museums, etc. I'm also concerned that September seems still to be the "peak" season, but we can't travel in October. So Question #1 - Would 3 weeks be enough for this type of itinerary; #2 Is September truly a packed month? We are reading Rick's Croatia-Slovenia-etc Tour suggestions, but we'd appreciate the views of those who know these countries. thank you!
Second half of September would be my ideal time to go, personally. September won't be as busy as the summer but because of the increase in European tourism in general, I would expect things still to be pretty busy especially on the Croatian coast. I wouldn't want to go in the summer, but I wouldn't hesitate to visit this region in September at all.
Three vs. four weeks? Personal preference - depends what you want to see and how long you like to spend in various places.
Put together a dream itinerary of all the places you want to go and see what you can fit into three weeks. If there are still too many things you want to see left out or too many short stops, add the extra week if you can. Personally, three weeks is about my limit for any trip before I start to get fatigued and homesick - but again, that's personal preference, nothing to do with visiting this region.
I think you'll find you need four weeks unless you're prepared to ignore the Croatian islands and sort of rush things.