Hi Matt, You're doing a very similar trip to my forthcoming trip in August. We will be heading from Suvarnabhumi to Khao Yai, then Khao Yai to Buriram, Buriram to Koh Chang, Koh Chang to Pattaya, Pattaya to Bangkok and finally Bangkok - Khao Yai - all except for the last leg, in a rented minibus. Those buses are a very good choice and reasonably priced when you consider how many passengers you'll be carrying. I've rented one before. If you haven't booked one yet, book it as soon as possible - not so many companies have them without a driver and those that are available sell out fast.
There are plenty of places to visit on that route - depends on what you like. The area around my home in Khao Yai has plenty to see and do - not least the National Park, despite the crazy farang entry fee but that would more than a 'stretch your legs' visit. There's also Choc Chai Farm on highway 2 before Khao Yai/Pak Chong and a few miles further on Lamtakong reservoir which has service areas and viewing points.
Depending on your route, there are some good viewpoints on the roads over the mountains from Buriram down to Trat and don't forget, the route passes close to the Cambodian border and I seem to remember a massive market close to that.
Again, depending on your route, there are lots of places to visit on the road from Trat to Rayong. I can't be specific but there are blue tourist signs all over that road. I believe many of these are craft centres.
Hope this helps.