They usually have balloons at the check-in desk. Nothing fancy, just some latex ones. Some resorts, either LS or check in counter person might know some balloon animal tricks. I've seen them do it at POP a time or two.
I would think a special outfit for the first day in the park laid out on the bed would be cool too. Along with some of their theme park gear, like an autograph book and pen. You can also check with the front desk at check in about getting a few Disney Dollars.
Streamers would be easy and light to pack away in the suit case. And a pack of latex balloon would work best brought from home, and a mini hand pump if you're short winded. Kids have fun with balloons on the ground as much as in the air.
Ohh and some window clings. People love seeing them as they walk past, and the kids will know which room is theirs.