On a hot day like today, take a wander down to the National Mall, and I guarentee you'll see some prime tantrums being thrown. "It's too hot!" "My feet hurt" "I want ice cream" "I don't care what's in that building, I'm sick of museums!"
And this is just the parents.
So, before your kids turn into a molten pile of goo and your wife gives you that look, take some time to relax, buy them (and you) some ice cream, and maybe even splash around a bit. Here are some options: