Beat me to it.
Definitely a camera, and a good pair of shoes.
Sunglasses, maybe, but not at that time of the year. It won't be hot, but you'll still get the sun. It's more cloudy between now and March than it is throughout the rest of the year.
It really depends on what you're planning to do there. If staying around Las Vegas Blvd, definitely a good pair of shoes, camera, headache/migraine medicine (for being drunk/loud shows/partying). Bath salts, rose petals, and empty suitcase. Bath salts because not only will it and the rose petals be romantic, but think of how much walking you're going to do. The CAT bus going down Las Vegas Blvd. is always packed at $2 per person, and taxis are worse. Sometimes, it is better to walk (though the monorail may help out better).
If doing some things off Las Vegas Blvd, I'd suggest the sunglasses, the camera, and honestly, some carrots or sugar (get them when you're there). It's about now that the mustangs, mules, and the like are roaming free around Redrock Canyon so when you're out there on the scenic tour, you could feed them. Same with Oatman (donkeys run the town; seriously).
Perhaps keys to a (rental) car? Trip to Route 66?
BL.