If you are awake at all during the day and like old historical stuff, do not miss the Neon Museum: http://www.neonmuseum.org/
If you are up for a drive and have never seen it before: Hoover Dam.
If you are with your lady: Cirque du Soleil Zumanity (don't see this alone; you'll regret it)
Some of the best Japanese food I've ever had was from Ichiza in Vegas Chinatown. It's worth the wait. And it's off the strip.
If you're on the strip, need diner breakfast foods stat, and have always wondered what it would be like to be inside Boogie Nights: Peppermill Restaurant.