I'd go for pearls. Not necessarily Tiffany pearls, but a beautiful pearl necklace with a lovely clasp. This piece will serve you well forever and will always remind you of your mother and your graduation. Over the years you will have many handbags, but that first strand of pearls is really meaningful and you wouldn't believe how much use you will get from them.
Just my own opinion. I still wear my 21st birthday pearls which I received from my grandparents in 1973. My bags from 1973? who knows what ever happened to them.
--Judy