When I get too hot I like to hang out at the library. Today the Boston Public Library West End Branch is showing two documentaries about the history of Boston starting at one PM, could be good to catch.
The city of Boston has also released a list of pools you can take a dip in for free during the heat emergency. Find that here: http://www.cityofboston.gov/news/uploads/19643_31_19_24.pdf
For me personally, I will be sitting in the AC in the living room today, listening to Pandora. I might even make some sun tea on the porch.