Talk to the venue operators and see if you can return the tickets for a show credit or cash. Special circumstances involving illness or other unforeseeable problems may entitle you to a refund.
List the tickets on craigslist.com. Create a listing that contains the show title, the show time and a way of contacting you. Never sell tickets for more than their face value. Trying to make a profit by selling tickets is considered scalping and is illegal.
Use the tickets as a tip for a host, waiter, housekeeper or bartender who has given you excellent service. Just because servers and other hospitality-industry workers spend every day in Vegas, doesn't mean they wouldn't enjoy a night out on the town.
Donate them to a stranger. If all else fails and you are unable to get a refund or credit for your tickets, consider making someone's day by giving them free tickets to a Vegas show.