I don't know. It's a real issue as obviously one doesn't want to make other parents, especially relatives, feel unhappy about their child's development. No one benefits by discovering just how far ahead your DD is. And yet one doesn't want to deny your DD activities or shush her up, that wouldn't be fair to her.
Thanksgiving was a bit of a trial for us as DS2.6 was doing some things a 4.5 year old relative there wasn't yet. The grandma was playing a academically oriented game with the four year old and actually said, "Come here (to my DS), X (the 4 year old) can't do this but I bet you can", in front of X. Of course DS did it, luckily the X's parents weren't in the room, and X seemed to not be paying attention. Could have been a lot more awkward.
Instead of trying to hide DS's abilities we just complimented the 4 year old and the parents on everything we could think of. Kid running in circles: "He's so fast, wow, he'll be great at soccer.", When he was whining to the point the parents got angry, "He's not one to give up is he, I wish I had his persistence". "Wow so he comes when you tell him to, what's your secret?". Etc.
We did save his maze book for times when the relatives weren't around. Still the grandma saw us doing it and commented and we said, "oh we do mazes together" (technically true, it was a tiny little one too small for his fingers, so he says which way and I draw the line).
We breathed a sigh of relief driving away to tell you the truth, next year is going to be worse.
As far as things you bring maybe just bring a big range of levels of everything, throw in some books and puzzles that your DD has outgrown (down to peg), that will make your DDs level faintly less obvious and give the ND cousin something to play with too. We have a couple of peg type puzzles DS still likes as he uses the figures/animals for characters for pretend play, maybe you have something similar.
There are activities too that don't have too clear age boundaries, by planning carefully ahead you may be able to bring some open-participation things. For example, making puppets from old socks, the ND cousin will be able to choose some items for you to glue on the sock. Probably some Xmas ornament crafty things would be open-age too, can't think of what at the moment.
Good luck, we're staying home for Xmas!
Polly
