The botanical gardens at the Bellagio conservatory is one of my favorite places to visit when in Las Vegas (not to mention it's conveniently on the way to Jean Philippe's patisserie ;)
Each season is a new and breath taking display. It's open 24hrs, 7 days a week and provides for a great photo op. There's tons of tourists here but it's worth to drop in for 10 minutes if 800+ pound pumpkins or beautifully decorated Christmas trees interest you!