If I rent a car in Prauge, which eastern european countries can I trave

Prague is a city in the Czech Republic, and Slovakia is also part of the former Czechoslovakia, so at the very least those won't be a problem. To be sure, ask your rental agency what they permit.

It really depends on the agency. As for ease of travel, Slovakia, Hungary, Austria would be the simplest since they are close. Romania, Croatia and Bulgaria are quite a bit further. Road conditions in all those countries are good, Bulgaria being the least developed.

I guess it depends on your time and driving stamina. Why not take the trains though? They are extensive, frequent, and will leave you far more rested than driving. Or even a combination. Drive in the Czech countryside for a few days visiting small towns, then drop the car off in Bratislava, taking the trains from there. Tons of possibilities!