I have a DS13 and a lot of my ideas have all been suggested, with the exception of a game called Can Jam. It's a frisbee game that you play as a team and you have to get it into the hole on the can. He played it this summer at a friends house and asked for it. I picked it up at Olympia with a 30% coupon they had.
another idea would be a camera...waterproof, shockproof type.
My DS and his friends play Manhunt a lot too, year round, so flashlight, dark clothing...
a pocket knife? a good sleeping bag, duffle bag for overnights, skateboard, bike, gift cards to local fast food places