Hi~ We camped with my now 4 yr old every summer. We never used a playpen, but did try to bring an extra adult with us (my mom), which helped make the whole experience much more laid back for all. Here's what we found helpful:
* Kid friendly versions of the adult camping stuff - we made his own camping kit with a small orange plastic shovel, an old mess kit, his own flashlights, a few tent stakes, rope, a spatula, plastic binoculars, etc. He spent a lot of time mimicking whatever interesting adult behavior he watched ~ cooked on a'fire' made of piles of sticks, hammered tent stakes into the ground, etc.
* Lots & lots of extra clothes, so we never had to worry about him geting dirty or wet.
* Books, a few crayons & stuffed animals for tent games. One rainy day we just cleaned anything dangerous out of the car & he spent 2 hours climbing all over our station wagon & pretending to drive.
* Baby backpack or Ergo. Absolutely required for hiking.
* We diligently taught fire safety. Repeat, repeat, repeat: "The fire is hot. Not near the fire". Now we can have a fire & really not worry at all. We did the same with campground roads & the boundaries of our space.
Until last year we put our boy in the sleeping bag between us, now he has his own bag & little sleep pad. In the sumer we try to camp near a river or lake - instant entertainment. Also helpful--be sure to bring natural/kid friendly bug stuff & sunblock. Camp hammocks (a giant swing!)& child sized chairs are also awesome, as are harmonicas & guitars if you're musical. Enjoy!