I agree with you that 18 should be something "memorable" rather than cash or gas cards. Those things are certainly appreciated, but they're gone in an instant.
My daughter's turning 18 in a few months, and I'm giving her a very nice gold cross necklace. We gave her a nice piece of jewelry for her 16th birthday, and I'm pretty sure she's worn it every single day since then. I plan to get her a new Bible for her 18th too. If I had a son, I'd consider a very nice watch.
She'll graduate less than a month later, and we're giving her practical things then: I've been collecting some dorm items for her, and we're going to add a handful of restaurant, movie, grocery store gift cards -- things to take to college.