My now five-year-old son loves Mythbusters and wanted a Mythbusters birthday party. He invited 3 of his friends to the party. We did 7 myths in 1 hour and 15 minutes (that was plenty of time). I didn't really explain the whys, the kids didn't seem very interested.
Here is what we did:
Myth: a toy car can out race a sports car.
What we did: raced toy cars against bigger toy cars. Little toy car was a party favor.
Myth: A pirate's eye patch was to help with seeing below deck
What we did: did an obstacle course in the garage, then wore an eye patch for part of the party and went through the obstacle course a second time.
Myth: You can't fold a piece of paper in half more than 8 times
What we did: folded pieces of paper in half. I then showed them (with a piece of newspaper) the trick about folding height-wise until it couldn't fold any more then folding in half lenght-wise. I got 9 folds out of it.
Myth: You can't pull apart 2 phone books with interleaved pages
What we did: Interleaved the pages of two novels (would have done phonebooks, but couldn't find 2 of the same size) and let the kids pull. They eventually succeeded, mostly because I was lazy in layering the pages. It went through 4 sets of kids and 1 set of adults before they managed it.
Myth: Quicksand will suck you under
What we did: mixed cornstarch and water and let the kids see that moving their hand slowly through it worked better than moving quickly
Myth: Diet Coke and mentos
What we did: Bought a six pack of little diet cokes and let each child put a mentos in it. Then for the grand finale, I did a big bottle of diet coke.
Myth: Pop rocks and soda will make your stomach explode
What we did: decorated cupcakes with pop rocks, ate them, and drank soda. I also explained a person would burp before their stomach exploded.