We do a ton of Geocaching with the kids, lots of fun. Gets us outside hiking around looking for hidden treasures. We are even hiding our own caches now, kids love it.
We use an app I found through the App Store, it's just called Geocaching, second one down when you search, sorry I can't provide the link.
It's fantastic with tons of caches all over the world, we use it on all our travels, as far south as the southern tip of Baja. We upgraded and paid a bit of money so we could access all the caches and also be members so we could hid our own.
It never gets old and you never know what you are going to find.
Happy hunting
Cheers