If someone says „I don’t know“ in response to you asking them what they want for Birthday/Christmas/etc, get them a jar of nothing. It just needs to be an empty jar with something that says „I don’t know“. Then get them a selection of candy, soda, whatever. Have that sit aside and give them the jar of I don’t know for the present. Inside of the jar, put on slip of paper for each item. Have them draw out x slips of paper (don’t make it all of them, maybe about half) and that is their present. If you are close with the person, maybe you can take the other half of the items and have a movie date or night or something.

Note: for the items, they can vary in size/amount/price/etc. so you can have a Package of MnMs but also a $10 gift card to wherever. (If you want to do the movie night idea or similar, make it something you two can do together like a restaurant gift card or something).