Isn't Geocaching fun?! It is a great way to get some exercise and discover new areas in your community.
Even though walking might be tough on your knees now, I believe, they will slowly improve through the gently exercise. I like to take a walking stick with me, it helps with the uneven ground. A stick is also helpful for poking around bushes and rocks in case their is a critter in there you might not want to meet!
As far as trinkets go, I would aim for the kids. Most adults don't worry about it but the little ones get such a kick out of finding TREASURE! Go to the dollar store and buy a bag of rings or little cars.