I would translate it as a "treasure hunt" instead of attempting to find a cultural equivalent to the scavenger hunt.

Strictly speaking, I don't see geocaching as being much like a scavenger hunt anyway. The scavenger hunt is all about "scavenging," meaning that you are in a race to get a list of things without buying them. "Treasure hunt" is a lot closer to the intended meaning of the sentence, and I'm almost certain it can be translated literally without loss of meaning.