We're also doing OOT bags - 1 per room. I bought 21 a while ago cuz they were on sale & I figured we probably wouldn't have more rooms than that, but I have no idea who's actually coming yet so I'll have to figure that out later.
So far we have pashminas for the girls, emergen-c, shout wipes, lip balm, sunscreen, little first aid kits, water guns (hee hee, I couldn't resist) and will be getting sudoku books, advil/tylenol, pepto or imodium, & maybe some cards. I think for the guys we might get a mini tequila or shot glass once we're down there. And we'll get some little pool toys & activity books for the kids that are going.
I like the idea of getting things locally when you're there to add to the bags, then you don't have to worry about bringing it all.
And hitting up Costco is a really good idea!
I'm not so worried about getting stuff there intact cuz none of it's really breakable, but I am a little worried about lugging it all there! I've heard from multiple people that we should hold onto our receipts & bring those in case we get stopped at customs. Also might be a good idea to split things up between you & FI and any friends/family coming from your area.