On those extremely hot days, where you break a sweat breathing, find the nearest river, lake, ocean or pond and go for a ride. I recommend taking your old junker bike, (unless you *like* re-greasing bearings). I like riding in the river the best. There are nice shallow parts where you can get up speed for when you plunge into a deep part. This is great for increasing your technical skills, especially in rocky rivers. Don't try this in the Mississippi or the St. Lawrence, they aren't shallow enough to ride in.
Off the Dock
This is one of the funnest things I have ever done. The first thing you need is a dock, which goes out into a fairly deep lake (make sure you can swim to the bottom though) and has a sudden drop off. You will also need to know how to swim, and you need a rope. Build a ramp at the end of your dock if it is a floating dock which sits on the water; it's more fun if you get height. You definitely want an old bike for this, preferably a BMX.
Now get as much speed as you can an fly off the dock into the water! Make sure you abandon your bike before it hits the water, because it will slow down quickly. Now your bike is at the bottom of the lake. Take a rope and tie a loop in it, swim down with it in your hand and hook the loop under your seat. Now swim back up with the rest of the rope, pull it up and go again! Once you get experienced and get about 5 feet of height, I recommend going over the handle bars (it's kind of like a dive compared to a cannonball).
Catch a wave, get tons of speed along the shore and when you see a wave coming scream out into the water and try to ride through it. You definitely don't want to do this with an expensive bike as the salt in the water will corrode the frame (especially Aluminum). I swear it's worth buying a $20 disposable, fat tire cruiser at a garage sale though.