Note that some of these require Shockwave or other plugins to work, and probably a fast connection as well.
- Carve a pumpkin -- Be sure you wait to see it light up
- Shoot zombies--my personal favorite
- Scarecrows and ravens (not a terribly easy game to understand, but atmospheric)
- Turkey Shoot
Links courtesy of News from ME and Lifehacker (more links to other games included in the comments to the Lifehacker post)