Just as important as what you pack -- and you can check out get-ready checklistshere -- in my opinion, is how you unpack.
Do it immediately. As soon as you arrive (unless someone in the family is overly cranky or tired), set up your room to make it as close to home as possible. Put the baby in the playpen or hotel crib with a pile of toys and occupy an older kid with a coloring book. Or have your partner take the older sibling out to get the lay of the land while you unpack. Settling in will help you remain organized (and sane) throughout your stay.
Designate a baby-changing station. Bring a box of wipes, lay out a changing pad (I like to put a disposable changing pad on top of a hotel towel), and stack a bunch of diapers in one area. That way, you won't need to chase down the diaper bag when that first big poop occurs.
Create a play space... Stash toys and books on a low shelf or in a drawer, or keep all the playthings in one corner. Creating a place for your baby to play will make the room feel homey and keep it from looking like a disaster area.
...and a kitchen. Even if your room doesn't have a kitchen or bar area, establish a spot where you'll keep bottles, dishes, baby food, snacks, formula, and dish soap. Sometimes the bathroom is best if it has the only sink in the room.