Small gift cards like Dunkin' Donuts or the local coffee shop
Do they eat out? Gift cards, even for the pizza place.
A baking mix - like a cookie mix with a cutter or two, or Brownie-in-a-mug kit
Lottery tickets
Bookstore gift card or mini book light for reading when the other is asleep (or during a power failure!)
Travel stuff, if they do that - shampoo/conditioner bottles, small toothpaste and folding toothbrush, tiny deodorant or shave lotion (see the "travel size" section of the supermarket), sectioned jewelry bag, small detergent & mini-clothesline for rinsing out things in the hotel sink.
AAA membership - even if they just travel locally, they'll have someone at the other end of the phone for a free tire change or tow.
Senior visitor passes to local museum