Free: Contests & Raffles.
How many kids Josh?
Maybe have a display of the 10 essentials that should be in your pack for safety while hiking/camping. Sounds like a lot of fun.
a little bit on survival would help. Just the basics like don't split up if lost, no panicing, stay put at night, maybe instruct how to get out of the weather. If lost following water down stream.
Take a bunch of plaster of paris and have the kids take some animal prints in mud. They can take them home with them!