We usually have a senior dinner and then present gifts to them. When I was a senior, I got an engraved jewelry box - it was beautiful and something that I have kept. Its cool to give AXO stuff, but by the time you graduate, do you really need another mug or keychain??
This year one of our alums has graciously (been) volunteerd
to make bracelets for them. She makes awesome jewelry, so she is going to put something together along with a little charm - either a lyre or a butterfly for our local mascot. She made me one awhile back and I still love it
You could also do a senior roast - but only if people will be good natured about it -- its always funny to hear people talk about the good 'ol days.