When my husband and I visit the Strip, we almost always stop by the Fountains. It's a free show which makes for a cheap date night. But more importantly, the Fountains are beautiful the songs and choreography are so much fun!
This is also the number one thing we like to share with our out of town guests. Which is also everyone else's idea, so things can get crowded. I would suggest going early in the week to avoid crowds. And I would also suggest visiting after 8p when the shows happen every 15 minutes. You can easily kill an hour and watch four songs - it's so fun!!