I think at that age, your options are a little limited. At 2.5 years, my daughter wasn’t even that interested in playing with a fun new toy for very long unless someone was playing with her. And she was also the kind of kid who slept like a champ in her own bed, but being at someone else’s house was just too exciting for her to sleep, even if we tried to wear her down.
The best we managed was to provide some toys on the floor, and roll with the fact that she would play a little, and then want to show someone her toys, and let her rotate through the adults. If you go into it expecting to talk, but with a frequent interruptions to look at a toy cow, you can relax and talk around it. If you CANNOT relax into that mode, I think you’re setting yourself up for frustration. I think some kids can be directed to play independently at that age, but developmentally, most kids are not there yet and it’s not about discipline or manners.
As to actual things to do, music is a great idea (aka toddler dance party). Maybe an ipad with the volume low for her to watch something while the adults talk. At that age, most kids will be more engrossed with a movie or show they are already familiar with, despite our adult belief that a new movie will be more entertaining.
At my mom’s house, we used to set up a row of canned food from the kitchen and send her to fill a basket (toys that involve putting things in other things and then taking them out are perfect for that age, even if they aren’t actual toys but rather tupperware from your kitchen), but even with that it cycles through the kid playing on her own, and then the kid bringing her groceries to an adult for inspection. Related, giving her a purse or tote bag filled with stuff and asking her to pack and unpack it. A laundry basket filled with socks.
My daughter was also interested in looking at photo albums of people she knew. We were mystified that she was able to pick out my mom at any age (even my mom’s high school pictures).
Fisher Price makes cute playsets that come in a carrying case (the line is called Play 'N Go, and sometimes Fold ‘N Go) which are good for a grandparents’ house because they are very small, portable, and easily stored (they’re about the size of a lunch box). Those were good for up to 15-20 minutes of focused play. Perhaps one of those for the Christmas list.
We are also big fans of a walk, even just around the block, after dinner, and we are from a cold climate so we bundle up and have at it. If it ends with the child in a stroller, they might even fall asleep and then you can just park them when you get home. At Christmas, maybe a drive to look at holiday decorations.
(these are all sourced from me looking at pictures of my daughter at that age at family parties – this is what she was doing)
Oh, and SO many people told me that this was a good age for kids to string things on a piece of yarn, like dry penne pasta, or I guess anything that has a hole in it. My kid was strangely delayed in this area because it always involved crying for us, but apparently plenty of other people find it a good time-consuming activity for toddlers.