M.!.
We do the library and "story time" on Tuesdays which is free and kids do a craft with it. Also not sure of your finances, but we get a COSI membership every year. On the 3rd floor this is a great play area perfect for your kids ages. Plus in the summer it is air conditioned so on those HOT days it works well for us too. Other than that, we do Tuttle Mall and then we have our kids signed up for gymnastics as well.
Good luck and Have fun! I look forward to reading the other posts for some good ideas as well.