If you haven't letter boxed, go do it now.
It's a bit like geocoaching but more fun for little kids. Basically, you print out clues that lead you to a hidden box. Each box has a journal and a stamp. You also carry a stamp and journal. You stamp the box's journal with your stamp and stamp your journal with the box's stamp. It's interesting to read the box's journal to see how many other people have found the box before you.
Eventually your journal becomes full with unique stamps as most are hand carved.
Some of the clues will take you on a bit of a hike. It also introduces you to new places around town that you many not otherwise visit.
Atlas Quest is a nice website because you can easily track the boxes you have found. There are many great features on the website that make it easy to find boxes.
There is also Letterboxing North America that is an ok site. It's just hard to keep track of what you've completed.
These boxes are all over the place which makes it easy to find one on just about every outing.
We're planning on planting our own box this summer. The kids are looking forward to writing the clues and carving their own stamp!