Marathon board games?
Monopoly can be a vicious sport! I was once involved in a game of LIFE! that went on so long (it actually may still be going on, I dunno where that box is, heh), that I ended up with kids scotch taped in rows around my car I had so many. Scrabble doesn't go as long but you can do whoever reaches 5,000 points first wins or what have you.
Keep a chess game going always with someone waiting to play the winner...
Also old-school card games are fun. Cribbage, bridge, gin, poker, hearts, whatever.
Plan and make a picnic dinner (for outside or, if it's too hot, move the living room furniture, get a blanket and voila) - plan the menu, go shop, everyone is in charge of making something and you all work together in the kitchen.
You can do other just 'it's hot' stuff, like making a bunch of -ades and then making popsicles. Go to the store and load up on lemons, limes, oranges, pineapple, berries, yogurt, pudding mix, whatever, come home, make limeade, pitchers of whatever, blend other fruit, mix some with yogurt, make a big batch of popsicles. Everyone devises their own flavour mix and gets however many spots in the molds for their concoction. If you don't have popsicle molds, basic ones are really cheap and last forever.
You can have story day - say you'll each think up one starting sentence or topic and then everyone will write a story starting with that sentence. Even teens will get into it, especially if you start writing stories from your childhood or from when they were little, they'll be interested.
If you just make it like 'it's hot, I'm bored, let's be goofy' even the 'coolest' teens will usually be swayed into it when they see everyone having fun.
