Whenever you're looking at a camera accessory, the biggest question is how much do you want to spend?
Depending on budget, I have a few ideas. One of the most useful accessories you can buy for under $20 is a lens pen, like this model from Nikon. There's a felt tip with a dry carbon cleaning compound on one end, and a retractable soft brush on the other. Camera lenses can pick up dust like magnets, and just one big speck can ruin an entire day's shooting. If your budget is even lower -- say $10 -- you could pick up a memory card wallet that'll keep your cards nice and organized rather than lying in the bottom of your camera bag.
Moving up in price a bit, consider a speedlight flash. There are a ton of them on the market right now, and you can pick up a decent one for $40 to $70, like this model from Yongnuo. They can subtly fill in shadows, even outdoors (especially outdoors in bright sunlight). When skillfully used, no one will even know you used a flash.
Speaking of, if your partner doesn't have a camera bag, or is looking at getting a new one, why not consider a sling bag like this model from ThinkTank if you have a bit more budget? I have one, and it's a lot more handy than a backpack or even shoulder bag for quick shooting. It's even handy for indoor shooting, as you can keep everything with you and quickly change lenses. In the same price range, consider a neutral density filter like this one from Tiffen. It's more for outdoor than indoor shooting, letting you shoot with with a larger aperture on a bright day to better isolate your subject from the background. There are cheaper models out there, but you do get what you pay for.
Finally, in the $100 to 200 price range, which is admittedly a pretty generous gift, why not consider a tripod or monopod? The Befree lineup from Manfrotto can handle both video and photos, and you can often find them on sale. If you prefer to shoot quicker, a monopod might be in order to provide a stable base, but not a complete movement-free platform.