I know it gets tiring keeping up with kids, but I think you're extremely lucky that your kids want to be OUTSIDE and PLAYING rather than watching movies.
Now, having said that :), here are some options:
1. pine cone, log decorating - bring glue, glitter, etc. and have the kids collect moss, etc. and then they can sit at the picnic table decorating.
2. photography - get them each a cheap digital camera and let them at it. Give them assignments like "take a photo of wood" and encourage them to use their imagination (a twig, tree, bench, sign, etc. could all meet the category wood). P&S digital cameras are great for macro photography, so encourage them to take photos of mini flowers in the grass or bugs, etc.
3. scavenger hunt - make a list of natural items that can be found around and in a campsite and have them search for them.
4. let the kids plan and make a campfire meal. There are tons of recipes out there (not just hot dogs and beans anymore). You can start this before the trip and let them help identify the ingredients, etc. The night of the meal, let them do the preparations and everything.
5. get them to help in the set up and daily chores.
6. enjoy this time with them. Soon enough they'll be not wanting you around at all and you'll be missing the days of watching them play in the lake.