I agree, themed days are usually more fun to do. We did a luau theme one year which was really cute. Everyone got lei's, we had lots of fruit and summery food, the New Member's bid bags even had little bears (one of our mascots) in grass skirts in them.
I've also seen everything from Army themes (Be All that You can be, be XYZ) to make-up themes (Maybe She's Born with it...maybe she's XYZ). You can make almost anything into a bid day tshirt and theme if you really want to!
