A couple years ago, my mum's boyfriend filled a shoe box with household essentials - loo roll, tea bags, that sort of thing - and gave it to my mum as a mother's day present. He deliberately put all the stuff in there that he knew we always forgot to buy. She teared up and said it was one of the best presents she'd ever had, because of the humour of it and the thoughtfulness, the fact that it showed he knows her so well as to know the things she always forgets, and to be there for her. Very simple, very sweet, and very meaningful present.
My mum also loved when me and my brother took her to see Rocky Horror live in Edinburgh. She's loved it for years and years and couldn't afford to take us all, and it showed how much we know her to take her to see one of her favourite shows, particularly after she'd been so distraught about not having the money to go. That one cost money, but what meant something was the thought, for her.
That's the sort of thing that makes the best gift, imo. If there's something she hates doing, do it for her. If there's things she forgets, get them for her. Show your appreciation by supporting her, and showing that you know her. ^^ so I'm gonna ditto
@LunaValentine - one thing you can do is cook her favourite meal, so she gets to relax for the evening and gets something she might not get very often. Spend time with her, doing her favourite stuff.