I am a huge fan of rare signed first edition books, particularly related to one's interests. I have given rare (not necessarily signed) 16th Century (or earlier) medical books to doctors. I have given rare books to friends who love athletics and athletes. A little footnote: When one of our sons was turning 18, he obsessed about that birthday. I knew that there was nothing (short of a sports car) which would please him. I also knew that Catcher in the Rye was a book he truly loved. I went to Heritage Books in Los Angeles and bought a true First Edition of Catcher in the Rye. He was dazzled! I might add that he is now over 30 and his 18th birthday gift has turned out to be quite a great investment. In fact, he keeps it in his safe deposit box.
There are a wide range of great First Editions suited to anyone's taste. Love flying and space? A great gift is a book written and signed by an astronaut (e.g. John Glenn). A good source is www.abebooks.com (do advanced search for signed 1st Eds).