It depends on what you're gifting it for! Assuming it's just to have around the apartment/visiting the Parks/running errands, a Stanley Cup or an Owala in their favorite color is a real treat. The Stanley is very popular, but the Owala will probably be easier to carry around the Parks.
If you want to gift them something they can carry while they're working, that's when things will get a bit more complicated. The Mouse will give them a standard water bottle and holder that clips to their costume (plus they can easily get this replaced if it's lost) but some people like to bring their own, since the quality of the plastic bottle isn't great. If you're looking to gift them something that they can carry with them on the clock, you'll want something that's very small, lightweight, and has a cap connected to the water bottle that won't get lost. Look for neutral colors (clear, black, or white will be the best).