The Gondola Ride at the Venetian is fun to do once.
It's a 10 minute ride down the indoor canals of the Ventian Hotel in Las Vegas. Tickets can only be purchased on the same day and it is $18 a person. There's a maximum of 4 people on 1 gondola and if you want your own private gondola for 2, it is still $72.
As you sail down the canal looking at the shops and bridges, the gondola paddlers will even sing during the tour. This tour is a little cheesy, but can be romantic. If you feel like burning some money, you might as well try it once. It's better than throwing money away at the casino table games, although some people will disagree with me.