I like the scrapbook idea too! Shutterfly makes amazing scrapbooks. Or a dvd with photos.
Do you have any jewelry or something you recieved for your 18th? My mom always did that for me. She gave me necklaces she had recieved, I loved it because it had a deep meaning going from her to me.
Another idea is put together a CD of songs that remind you of her or times you have had together.