Ask your mother what she'd like to do!

If she says, "Nothing" or "It doesn't matter" then use your brain and think of something you know she'd like.

Does she like the outdoors? Gather the family and have an old-fashioned picnic.

Is she always doing for others but never takes time for herself? Give her a day of pampering for herself and a companion.

Above all, be sure to tell your mother how much you love her and how much she means to you. If you put that in writing - in your own words - I suspect that would be the best gift of all.