1) Chalkboard Pumpkins - Here's a great alternative to the mess of carving pumpkins that you'll just have to throw away in a few days. Working outdoors, cover pumpkins with two coats of chalkboard spray paint. Allow 24 hours for them to fully dry. Next, divide your youth into small groups, provide them with some chalk, and challenge them 1) to answer pumpkin-related trivia, requiring them to write their answers on the pumpkins, 2) to draw on their pumpkin the best likeness they can of the face of one of your youth leaders, or 3) Have a pumpkin design contest.
2) Pumpkin Brooming - Provide participants with sturdy brooms and one pumpkin each. Enjoy the fun as they use the brooms to race the pumpkins from one end of the room to the other. For added challenge, set up an obstacle course for them to manuever through with their pumpkins.
3) Mummy Wrap - Divide into groups, have them each select an "it," and provide each group with several rolls of toilet tissue. Shout "Go!" and see which group can be first to completely wrap their "it" person in the toilet tissue. If you don't like the mummy reference, pretend you are reenacting the Lazarus story.
4) Night Bowling - Just drop some glow sticks into 12 water bottles (with the water still in them), screw the caps back on, set them up like bowling pins, grab a ball, wait until dark, and see who can be first to get a strike!
5) Hell House - Though I don't suggest you actually take your youth to one of those Christian hell houses, you can spark an interesting discussion by watching together the documentary "Hell House" and talking about the particular approach to evangelism it depicts. (Note: I happen to think the kind of hell houses displayed in the film are a terrible idea but it is a great documentary).
6) All Saints Prayer Stations - Halloween is really just a precursor to All Saints Day when we remember those of the faith who have gone before us. Consider inviting your youth to participate in an All Saints worship experience by adapting some of the prayer stations found here.