When you look at a camera accessory, the biggest question is how much you want to spend.
Depending on the budget, you have a few ideas. One of the most useful accessories you can buy for less than $ 20 is a lens pen, like this Nikon model. At one end there is a felt tip with a dry cleaning agent for carbon and at the other end with a retractable soft brush. Camera lenses can absorb dust like magnets, and a single big spot can ruin the whole day shooting. If your budget is lower – say $ 10 – you could take a memory card wallet that keeps your cards nice and tidy instead of lying in the bottom of your camera bag.
A little money in the price, Consider a flash of lightning. There are currently a ton of them on the market, and for $ 40 to $ 70, like this model from Yongnuo, you can buy a decent one. You can subtly fill shadows outdoors, especially outdoors in bright sunlight. With skillful application no one knows that you have used a flash.
If your partner does not have a camera bag or is looking for a new one, should you think of a sling bag like this model ThinkTank, if you have a little more budget? I have one and it is much more practical than a backpack or even a shoulder bag for fast shooting. It's even handy for indoor photography, since you have everything with you and can quickly change the lenses. In the same price range you should use a neutral filter like Tiffen. It's more for outdoor shots than for indoor shots. So you can shoot on a bright day with a larger aperture to better isolate your subject from the background. There are cheaper models, but you get what you pay for.
Finally, in the price range of $ 100 to $ 200, which admittedly is a pretty generous gift, why not a tripod or a monopod? Manfrotto's Befree product line handles both videos and photos, and you can often sell them. If you prefer to shoot faster, a monopod could be to provide a stable base, but not complete freedom of movement.