Well, here's one thought...
If you have access to a computer lab, or even one computer and can project the screen, go to:
Now, choose an open classroom for your appropriate grade level. Voila! You have access to a google-map driven, world-wide scavenger hunt on your computer screen! The hunts have been created by terra users, as well as the web site. Try the tutorial first, then the others. If you like this as a good, back up activity, you can do what I did and register (for free) as a teacher, then set up a class in which you can capture the best hunts you find and make them available for your students via a username and password. Check out my 5th graders hunts by using the username: educaching, password: woodland
Okay, this is one rainy day idea. Now it's your turn! Add another GPS/map related idea you could use in lieu of inclement weather!