I loved camping as a kid, and I'm excited to take my three year old on his first trip in a few weeks.
One activity kids often enjoy is gathering collections. As teacher, I often start this game as "let's make a campfire" and then begin putting small sticks into a pile. Usually the kids will either join up with me in building a pretend campfire, or they'll make their own. (This usually indicates to me that they need a little downtime, and so this gives them space while keeping them safely close.) If you are camping at a place that's short on sticks just use your imagination: rocks, pinecones, pine needles, acorns, dandelions...the idea is the collection, not a perfect product. Campfire play can go on as long/short as you or the kids like and is a perfect idea for those "I don't know what to do" moments.
You can bring a few action figures or dolls that can get dirty and have the kids build 'houses' for them.
You can have the children use sidewalk chalk to 'mark a trail' from one spot to another. Good time-user and motivation for short walks, because they can then have the fun of following their trail back.
Marbles are fun. Buy a bag and plan on losing some. If you don't have a marble shooter, they can move them with sticks or use them for whatever suits their imaginations.
A length of rope is great. Tie it in a loop and leave it on the ground, see what the kids come up with. Jump ropes-one for each child- work too.
Bring a basket or bag for each child to collect small treasures in. You never know what they think is a treasure.
If you have a book that labels the flora and fauna, bring it along. We love our Audubon guide to the Pacific NW...lots of great pictures to keep the kids busy and helps with identification. Usborne, too, makes some great educational books. If that doesn't work, you can always go online and print out pictures of animals and easily-identifiable plants and staple it together for a book. There are also laminated naturalist cards to be had.
A new pad of paper for each kid and some markers or crayons is always helpful.
I hope this gives you some ideas. Dump trucks and shovels, etc. are also loads of fun... I saw a lot of good suggestions posted. Just let them enjoy the nature and I hope you have a great time.")