Since it's been hot, I've been staying indoors as much as possible during the day and going out for long walks after dinner. The air isn't that much cooler but at least the sun isn't beating down. I get my 5 or 6 miles and head home in the darkness.
Hot summer evenings in Back Bay are ideally spent watching the sunset on the Charles, strolling around the Public Garden (where the air is always a few degrees cooler), or having dinner outside on Newbury Street.
A less-traveled spot to sit and relax on a warm evening is the courtyard of the Boston Public Library's McKim Building:
A splashing fountain always makes the air seem cooler and the Roman architecture, landscaping, and lighting make it one of the loveliest free public spaces in Boston. You can sit and read, use the free wifi, or do absolutely nothing — in relative solitude — until nearly 9 on weeknights.
If you still don't feel like going home, head to 338 Newbury Street, where the Trident Bookstore and Café stays open until midnight. It may not be as picturesque as the old wing of the BPL, but both the food and the book selection are very good. (Plus it's air-conditioned.) Sadly, they are now "Boston's only independent general bookstore" so they deserve our patronage more than ever.