A fun and simple way to do this would be to create an account with StickyBits.com and place StickyBit stickers around your area requiring your participants to check in with a photo of themselves in front of each location. Stickybits are barcode stickers which allow people to scan them with their camera phones and add just about anything to the digital archive attached to the barcode; Pictures, audio, URLs, etc. You can purchase 20 Stickybit stickers on Amazon for less than $10.
Use a GPS code or clue to drive them to the first location. Once found, they must check-in with a photo of themselves at the location and then are given the code/clue to the next location.
You could also create a Fourquare Location (ie Farmville GeoContest Location 1, etc.) as Stickybits allows Foursquare and Twitter check-ins. I would create a hashtag on twitter as well, so when they check-in, they can include the #, generating further excitement as the game proceeds.
The Scavenger Hunt could be promoted to local tweetup and meetup groups, on your local facebook page and community online newspaper. Think of the fun and excitement a weekend afternoon scavenger hunt could create.
The good thing about Stickybits is that the code has to physically be scanned, no cheating like you can on Foursquare.
Other uses of StickyBits? Attach to your listing flyers, business cards, single property websites, yard signs, etc., you are only limited by your imagination. Consider a customer appreciation prize for the agent who checks in to the highest number of your open houses, or the buyer who visits the most number of public open houses… what do you think?