He's always game for a trip to the aquarium...
The Walters for you locals) which is incredibly kid friendly and interesting enough for many, many return trips (in future heat waves and blizzards).
(The museum hands out sketching kits to the kids when you enter, this made Zach and daddy very happy as they spent all morning copying hieroglyphs from various items in the Egyptian art room.)
(And more sketching of the bugs of course.)
(This was his favorite- the Faberge Eggs. It's now on the top of his holiday list! What every 6 year old wants, right?)
And of course, when all else fails and mommy doesn't have the energy to go anywhere, this is an easy way to beat the heat at home.