Bounce house/trampoline park...we have a couple of huge warehouse sized places around here. One is full of bouncy houses and slides. The other has trampolines built into the floors/walls and foam pits. I'm going to try the trampoline one tomorrow for kids hour. (6 and under)
ETA: A couple of our malls have indoor "parks" too.
Fill a water/sand table with things like dried pasta, pom poms, oatmeal, etc. Get some ball pit balls and make a ball pit with a pack and play or toddler pool. Make some bathtub finger paint and let LO go at it in the bathtub or toddler pool. Tape paper to the kitchen floor and give LO crayons to color. Our basement has a hard floor and I just put DS's ride on toys down there and he loves to drive around and around.
Re: Indoor Activities
5 DIUI - BFN
IVF#1 - BFP - AJ 7/12
FET#1 - BFP Due 7/24/14
ETA: A couple of our malls have indoor "parks" too.