A first aid kit with a little of everything for those of us who get sick every time we leave home. I make sure there is asprin, tylenol, benedryl, ammodium ad, moleskin for feet, alcohol rubs, basitracin, sewing kit, vitamin E for blisters, bandaids, etc. I also get a 6 pack of target travel tissues (cheaper than kleenex) and put a pack in the rental car and a pack in the camera case. I usually only take 3 or 4 of the packs and put the rest in the car at home. DH has allergies year around, so we always use them.
The other thing that goes with us every where is our collapsible cooler. It fits in the zippered "lid" of our suitecase and takes up no room at all. If we need extra luggage to get home, we use it as a carryon.