I think I'll mount the GPS to my horn today.

I've been experimenting with a GPS to mark the trails but the Garmin 305 Edge does so much more than that. Combined with www.motionbased.com software it will keep track of your mileage, the weather, wind, temperature, heart rate (for you or your horse depending where you put the sensor). You can view your trip on a road map or on Google Earth. Here was the ride that Lisa and I took yesterday on Roger and Cocoa:

http://trail.motionbased.com/trail/activity/2339952