Batteries, tape, hot glue gun, scissors. I always need them and usually end up lending them out as well.
(and yay now I know where to go borrow said items
Haha!! Exactly what I was about to say!!
Right on the money. I'd add to that some safety pins, and loads of bobby pins for those with long hair and wig plans. I give an ninth to earplugs and a second to your passport.
Also, I am very serious about my "feel better" kit because it's a marathon, not a sprint. We always bring stomach meds -- immodium, pepto, tums, gas X etc. There's nothing worse than an upset stomach when you're trying to dance. We bring aches/pains meds like Excedrin or Aleve because dancing for 16 hours straight is hard work, stuff to help get to sleep like Valerian, and stuff like Emergen-C to put the vitamins and electrolytes back in the next morning.
If you're traveling in a couple (or not and just have big plans for meeting someone), without putting too fine a point on it, I'd plan appropriately for the fact that you're on the vacation of a lifetime and will likely be feeling amorous.