If your hotel is anywhere near a beach, a beach towel. A friend got one from 3Com (the computer networking company) a couple of decades ago. It's gotten a lot of public exposure.

The thing that would get me to stay at your hotel, though, isn't a give-away. It's a pillow. The overstuffed ones I usually find look great on the bed, are fine for back sleepers, may be OK for side sleepers, but don't work for those of us who sleep on our bellies. Most places don't even have thinner down pillows available on request, let alone in the room.