We do a lot of traveling with Wee Child and some of the things that have made my day have been:
A small basket of age appropriate toys -- it doesn't have to cost a lot. Just a few small somethings
A basket of baby friendly toiletries, including wipes, diapers, powder, etc.
I like to leave a small tray with cookies (usually a tin with some I've baked), bottled water, and usually one, eat regional food. Here in the South, I leave some of our wonderful candies.
Do find out ahead of time if any one has health concerns, special dietary needs, etc. Also, find out if there's something specific they want to do.
And, like everyone has said, relax and enjoy yourself. Plan ahead and give yourself plenty of chances to relax and not stress out.