Caribiners! You should always have a few of these. They come in handy for things like climbing (obviously) but also self rescue, constructing shelters, hanging or repairing gear, etc. You don't have to have the locking type, although they are the best, but just a few of the climbing rated ones are a good idea. Don't waste your time with the "snap rings" that are used for key chains and say "not for climbing". They're junk.