My mom loves flowers and cards and things like that. She likes to do things with the family so usually we'll plan something or take her somewhere.
Most moms don't expect anything at all on Mother's Day, so really any small effort that you put in would make her smile. Find something that she likes doing and give her something to do with that. For example, my mom loves gardening so I would get her some flowers to plant or maybe help her weed the garden or put down mulch or do some things that she would appreciate. Breakfast in bed is always a good idea, and any sort of cleaning that she would usually have to do.
Mother's Day is about appreciating things that your mother does for you and showing her that she is doing a good job. There's an endless number of small things that can be done to make her smile and make her day better.