You need at least two sets of clothes unless you like doing laundry in the nude. I assume your will be riding more than a few miles between friends' houses so you need your basic multitool, spare tube, patch kit, pump, etc. Might want to take a few toiletries so you don't stink too badly. Get some of the small travel size items at your local xmart. Take enough water to get you between refill stops and maybe pick up a snack along the way.

Other than that, take whatever else you want to have with you. Footbag, book, stuffed animal, big screen Tee Vee, etc...