Not a local, so I'm sure you'll get better advice from someone else, but until then I shall offer my two cents. I was in Oahu around this time last year with my fiance. We stayed at Honolulu County campgrounds - there are about a dozen of them spread around the island. Prices were around $10 per night (IIRC) but note that reservations are only by 3- or 5-day weekend blocks (depending on the campground). For one weekend we stayed at Kaiaka Beach Park and it was awesome, tent pitched right on the beach with tons of open space all around, low key and yet pretty close to Haleiwa town. Closest campground to the climbing at Mokule`ia Wall (the two nearer campgrounds were closed when we were there). We had a couple great mornings at Mokule`ia - the climbing is as good as any basalt crag in the mainland U.S., and the local/community vibe is just wonderful. Lots of friendly folks and fun, well-equipped routes to enjoy. Have fun dude!!!