Have you been to the Arboretum? It's awesome. My little kids love it because they can run and play. I love it because it's beautiful. They still have the Storybook exhibits up, too. Go early in the day because it does get hot there. Pack a picnic lunch. It's a great place.
Also, each week this summer we have focused on a theme that the kids picked in the spring. This week is vehicles. So we went to the Frontiers of Flight museum on Monday and made paper airplanes when we got home. We made boats out of styrofoam egg cartons on Tuesday and raced them in the little blowup pool and checked books out of the library about vehicles. On Wednesday, we talked about trucks. We sat in the front yard for an hour, the kids played with little matchbox type cars/trucks and we counted how many trucks we saw drive past. We saw a Fed Ex man, mail truck, Schwans man, construction truck, AT&T truck, etc. It was crazy how many we saw on our little street! Then we drew pictures of them and watched a video we had checked out at the library about construction trucks. Thursday we went to Hobby Lobby and found little airplanes and helicopters that we were able to paint. They loved that. Today, we're off to the train museum for storytime and when we get home, we're making a Twinkie train with twinkies, pretzels and cheerios. Good fun!
That's what we've been doing all summer. My point is that if you let your child choose a theme, something that they're interested in, it's easier for you to find things to do. Otherwise, it's just too hard to narrow down what to do each day.
Some of the other themes we've had our pioneers, cowboys, prehistoric animals, science, explorers/pirates, great authors, and space. The rest of the summer, we were on vacation.
Good luck!