A itunes card or movie ticket is an easy present.
I buy my nephew a movie ticket each year and he is over 25. It is hard and maybe you can cut out presents when they turn 18. My Mum stopped buying the girls Christmas presents once they turned 21 and is doing the same for the other grandchildren.
I buy for 2 BIL's children even though they are grown and some are married but we see them nearly every week but don't get anything for the ones we hardly ever see.

It's hard to know what to do