Learn to use a GPS receiver. Practise by finding the GPS coordinates of three things, eg a tree, road sign, park bench. Hide an item for friend to find after giving them the GPS coordinates, then swap over.
Handheld GPS devices
Something to hide
Pen and paper to record coordinates with - or just swap GPS units with them saved in.
Attached document is basic instructions for using the Garmin etrex 20x.
Give a quick explanation of how to use the GPS and allow everyone to interact with it.
One person or group (depending on number of GPS units available) places several items and records the coordinates of each for another to find, then swap and use the coordinates to find the treasures.