Objective: Create a geocaching course with 5 or more caches. Time the students to see how long it takes them to complete the course. (Most GPS receivers have a timer of some kind).
At each cache: Students answer questions on their own sheet of paper. Then assign a time penalty for each question answered incorrectly.
Bonus activity: Students find the distances traveled and use math to find extra info such as average speed, elevation gained/lost, etc