No ideas, but I'd love to see what other folks come up with, since I hit the big 40 next spring.
So far, I figure there will be the usual day-of sushi dinner.
Actually, I know a few people turning 40 soon. Maybe I should do a repeat of my 30th birthday gifts, which was to fill a priority mail box with a bunch of little toys from Oriental Trading Company - I called it the "return to childhood" gift pack. Or maybe I'll mail a bunch of black helium balloons. :-)