Think of it as a puzzle - you can get a LOT of little stuff in it! I swear you wouldn't believe how much stuff I can get into a shoebox when I pack 'em for my BIL when he goes to Iraq!! Get one from the post office & practice! You can pack your door signs (on the bottom or standing along one side), your fish extender (fold it or roll it up), trinkets to put IN the fish extenders, the over the door shoe thingy if you're one of those ppl (roll it up baby). What else. Ummm small costume accessories (jewels, belts, wigs etc). Incidental kids stuff that they may want.
Do you take your own beach towels? I don't know. I just know that you CAN get more in it than you think. Pack it TIGHT!!
Did you see that post from that one person who said they wear like movie quality costumes and they used to mail like 4 boxes?! THAT kind of crap is exactly why they don't want it going on any more!! I mean, seriously, who needs to bring a freaking hat that has such a big plume that it can't fit in a 12 x 12 box? Really. Oh, and the big boots he said don't fit. I'm sorry. They're gonna have to get over that one! I think that's nuts to get that carried away that you have to ship 4 big boxes of stuff for one night!
I am just relieved that we CAN still bring on beer & water & whatever as long as it's in a carry-on!!!

THAT was what I was concerned about!