Just don't go to the Indianapolis Speedway this weekend. They're having the US Grand Prix, so it might be a bit busy.
We liked the Zoo when we went 10-12 years ago. Kids always like zoos.
We've also been to the Children's Museum several times. It's very cool. Last time we went, they were having a big Norman Rockwell exhibit. Don't know if its still running though.
Another good place to take kids is Conner Prairie. It's a living history-type place. My kids loved it. http://www.connerprairie.org/