Adventure, you say? If you have kitsch doodads hanging around, and access to a GPS, you should try
Geocaching in your local area. It's a great thing to do by yourself, or with anyone you can drag along. It's a great goal-oriented outdoor activity, often leads to really neat places outdoors, or little-known historical sites. Many Caches also require you to solve riddles, logic puzzles, orienteering challenges, and so on, and you can get way-cool swag out of it!
Other forms of adventure may include spending an afternoon with various field guides, learning local plants, birds, mushrooms, etc., buying cheep laser guns and organizing a game of Laser Tag Capture-The-Flag with folks from school, or hiking into the woods and building a lean-too fort. For more sedentary adventure, you could spend an afternoon building a Purple Martin house out of hollowed squash shells, or assemble a Rube-Goldberg Machine from toys and household items and film it in action, or make a jar of Napalm/Thermite for burning something entertaining. You could also grab some components from Radio Shack and build some simple electronic devices, like a Supersonic Ear, or a Short-Wave Radio Transceiver, or practice photography, or pick up a music-writing program and fiddle around with it, or dig out a telescope and try to find a planet.
When you have 8.5 months of sabbatical, you could do any of a near-infinite number of things. I wish I had those sorts of days again... I spend 8 hours a day working, and 4 more at school for most of the week. Cherish the free time you have, while you've got it.