One of the gifts I received from my sisters on my 21st was a beautiful shiny flask with my initials engraved on it. It's come in handy on many many occasions -- usually when I'm walking around town and open containers aren't allowed.

I've since bestowed the same gift to friends on their 21st. Depending on how big you go and where you go to buy it -- it doesn't cost too much even with the engraving.