First thing's first, the party would be in our yard. Doesn't get cheaper or more convenient, but yes, I would hyperventilate a bit pre-party praying for sunshine.
My son wanted to actually sleep out in a tent with a couple friends (and daddy, of course!), so we set up our tent in the yard, which ended up being a hit. The only minor snafu with the tent was when my two year old daughter drove the John Deer gator into the side of the tent and ripped a hole in it. (My husband was not so pleased. Enter post-party ordering of tent repair kit
Organized games make me a bit nervous, but my friend brought her stash of burlap sacks for a sack race, which the kids loved. I borrowed Ashley's fab idea of using brown paper bags to create a backyard scavenger hunt. We also set out a bunch of yard games, balls, and the mini trampoline. With a swing set, sandbox, and a bean bag toss the kids had plenty to do during the party.
We used our fire pit to create a campfire. We asked everyone to bring lawn chairs with them to sit around the fire and put a bunch of roasting sticks out for people to cook their own hot dogs and marshmallows.
I made some easy party favors: marshmallows, chocolate, and graham crackers in plastic bags so guests could make their own s'mores at home. We kept the rest of the food simple—veggies, pretzels, and popcorn. With all the fixings for s'mores and the hot dogs, there was plenty for everyone to eat.
All in all, the camping party was a win. My son had a blast, and enjoyed the coolest sleepover of his young life. And now my daughter is already asking to do this for her birthday...darn successful DIY birthdays!