Bring a bottle of good booze that your onwing likes a gift for him/her. For example, I gave mine a nice bottle of Cardhu (I think I spelled that right) scotch, which he cracked open right there and we drank most of it. Not all guys do this, and you might want to be sure your onwing actually drinks, if not, cigars or something along those lines. You might have to tell a funny/embarrassing story about your onwing or your experiences with him/her, so think of something relatively entertaining. If it's out in town, find out if you're wearing bags or civvies, don't be the one guy that sticks out.
Watch out for the inevitable drunk ensign/2nd lt who boozes too much and decides to tell an instructor or 2 off. Seen it happen, never results in anything positive.
Oh yeah, bring a tie to have cut. The uglier the better.