The total orienteering device with compass, thermometer and radio direction finder. Can tell you wind speed and direction, pressure, relative humidity, and time of day. It also has an inertial tracker which plots your course as your move.
Detailed topographic maps (each covering 200 km square) can be bought for $5 each and loaded into memory.
Finally, for an additional $20, it can be equipped to check the NavStar system and plot your position on the Earth to within two meters.