A friend of mine told me that her parents used to take her to the mall on hot summer days to stay cool. At first i laughed at it...
but, its brilliant.
Another thing you can do is visit the library. In New York they have ACs, internet access, and peace and quiet.
Another option is going into a barns and noble and just lounging around for the day.
Public pools are also a free option depending where you live.
What else is out there?