Apart from the obvious(tent, airbeds, bedding).
We're only going for one night (stargazing) and we're only 10 miles from home, so we really can travel light.
Oh, and I probably should mention we're camping at my Parents' farm so if we forget anything we can just pop into the house. DCs are keen to do it at least a bit properly though, and it's taken us 6 years to pursuade DP even this far!
What a bunch of softees!