Geocaching is a fun outdoor recreational activity.
I do not know much about it. 
You can put an item in a waterproof container for others to find.
What kinds of places do you put them?
Bushes, buildings, anywhere the public can find it. They are logged and people find them like a scavenger hunt.
Where do you log it at? Is there a website?
Yes geocaching is logged online.
What do you do when you find one?
You take a picture with it or mark it off that you found it and then you put it back for someone else to find.
That sounds pretty fun. I should look into it.