I did Slovenia, Croatia and Bosnia in July 2002. I posted a detailed trip report in the "Europe" forum.
Slovenia is like Austria, only cheaper. Croatia is like Switzerland, only cheaper (but not much cheaper). Bosnia is an economic mess.
I would have no more concerns about going to Slovenia or Croatia than I would have about going to France or Germany. No carnets needed whatsoever in those two countries, the Slovenians didn't even ask to see my passport, just looked at my licence plate and waved me through. The Croatians spent 3 seconds looking at my passport, then waved me through.
Credit cards are accepted everywhere in Croatia and Slovenia for gas, road tolls, etc., Euros are also accepted everywhere, although each country has their own currency. If you need local money, you can find an ATM in all the usual places to get the money from.
Bosnia - I don't recommend it. Things are too screwed up there, hard to relax and enjoy yourself.