If you're not into gambling much, the other activities I would definitely recommend would be:

1) See the Cirque du Soleil show, Mystere, at Treasure Island. Expensive, but fantastic! And worth every penny!
2) Ride the Big Shot on top of the Stratosphere Tower - what a rush!
3) Go out to Red Rock Canyon and do some hiking or bouldering. There are various trails - some pretty easy and some that will give you a serious workout!
4) Take a quick trip to Buffalo Bill's on the California border to ride the Desperado roller coaster. Great ride!