Hello there jgmany! Just seeing this - I'm now living in Michigan :( so while you're thinking about the heat, I'm thinking about the coming snow and brutal cold winds. Do you like roller skating? It is indoors and a/c running. As a teen I worked at a rink and they often have reduced rates "adult skate" hours mid week or early Sat. As a teen we called it 'senior hours' now I think it's perfect!
There is always mall walking but I'm not a fan of the mall walk because the smell of the foodcourt will make me want some garbage food and defeats the purpose. Oh, just remembered.. our local schools would let residents walk in the gym after school if not in use and sometimes the pool (the couple that had pools anyway) Good luck to you