I run a middle school geocaching club. I purchased 5 explorist 100 GPS units a couple of years ago for less than $80 a piece. With 5 gps units, I take 10 students at a time and they have to share the device with another student. Our school is right next to a preserve that has 15+ caches within walking distance. We go out every other Tuesday after school for almost 2 hours. I have to keep track of which kids went and which caches they found so that the next time they go we look for different ones.
Kids love it!. I find that I often get kids that don't easily fit in with other clubs. They want to get involved in something but don't see themselves as athletes for sports clubs or brainiacs for the academic clubs. This is a great place for those students to feel connected to something at school and to develop confidence.