Being also from Indiana, we are particular fans of the Indiana state parks and make use of them for long weekend outings every year (32 ft fiver). There are several close to Indy, with Brown County (Taylor Ridge campground) being our favorite. If you like boating, there is Lake Monroe (now a state park, I believe), Lake Patoka, and a number of SRA's easily within a 100 mile range. Turkey Run state park is beautiful and also one of our favorites with the best hiking in the state, but be sure to fill your fresh water tank before you go as the local water has a very heavy sulphur content.
Heading north, there are a couple of state parks with nice campgrounds, including Pokagon off of I-69 close to the Michigan border, Chain of Lakes northwest of Ft. Wayne, and Oubache in Bluffton (nice bike trail there).
If you want to go a bit further, we really liked Hocking Hills in southern Ohio. We stayed in the Old Man's Cave campground, but next time will try the KOA there. Then there's always Mammoth Cave National Park for a weekend. Nice campground, but no electricity which could be a problem for you if you feel the need for AC... and it does get hot and humid there during the summer. Straight shot down I-65, though, and the cave tours are great.
Lots and lots of options for you to explore and you're in a great, central location to get to them all.