HT Games
Baby Picture Matchup
Ask the teens to email a picture of themselves as toddlers to you before the party. Put the pictures up on a wall or a corkboard. As guests arrive, have them write down on paper their guesses about which baby picture matches up with each of their friends. Near the end of the party, put each guest's name under the appropriate picture. Total up each teen's guesses. Give a candy bar to the person with the most correct guesses.
Doughnuts on a String
Tie doughnuts to strings hanging from a branch or a broomstick. Have the kids try to eat the doughnuts without letting them fall off the string and without using their hands.
Pass the Orange
Ask one of the kids to tuck an orange (or some other round fruit) beneath his or her chin. Have the teens form either a circle or a line and pass the orange back and forth without using their hands. If the orange falls, the two people who dropped it are out. Keep going until there are only two people left.
"Kitty Litter" Treasure Hunt
A grosser version, for older kids, of a classic treasure hunt game is to take a kiddie pool and fill it with bird seed (which can later be used to feed birds in the winter). Melt Tootsie Rolls in the microwave and mash them together to form pieces of "cat poop"; after they have cooled, add the pieces to the pool. Throw small plastic toys and other party favors into the pool of bird seed and Tootsie Roll cat poop. Hand the kids plastic scoops and tell them to find the treasure and avoid the poop.
Hang Ten
Lay down a blue tarp, play some Beach Boys tunes and have the kids pretend they're riding a surfboard. Encourage them to windmill their arms for "balance" and to make wacky poses. When the music stops they must freeze. Anyone who doesn't freeze in time is out; and they sit off to the side until only one person is left. The last person wins a Hawaiian-theme prize (a whole pineapple or a coconut-flavored bag of candy, for example).