GeoCaching is known as "Treasure Hunting" in my household. Lots of local parks in the area have caches. In some places the map looks like you can use them as stepping stones across the region.

We find it is a good afternoon time activity where there is some directed interest/goal for the kids to latch on to. The GPS (an old model Magellan) gets us near, then it becomes a challenge to find the hidden cache. Often we just sign the log book instead of taking something (TN/LN - Took Nothing, Left Nothing). Teaching the girls that the journey is its own reward, along with "don't stick your hand in that dark hollow log without checking for critters first".

Be warned, I have friends who got very hooked on geocaching, to a disturbing extent...