Here are a few Vegas activities you might enjoy, including some free attractions.
http://www.lasvegas-how-to.com/things-to-do.php
References :

Just like the others said, there is still plenty to do besides drinking or gambling. Try vegas.com cheepovegas.com smartervegas.com vegasbuddy.com spyonvegas.com. That should give you a good bit of info. You can spend a couple of days just seeing the free shows and attractions and walking through all the hotels. Personally I think you would have a blast and if you do go that would just make it a must to go for your 21st. Just go, you wont be dissapointed and you will have plenty to do.
References :

There are many things you can do in Las Vegas. Try this website: http://www.findlasvegaseventsbuytickets.com/.

You'll find just about every Las Vegas venue and can browse the schedule of events, buy tickets, find closest Las Vegas hotels, view maps and seating charts, etc.

In particular, here a couple Las Vegas attractions to try:

Mandalay Bay http://www.findlasvegaseventsbuytickets.com/search/label/Mandalay%20Bay%20tickets

Cashman Field
http://www.findlasvegaseventsbuytickets.com/search/label/Cashman%20Field%20tickets

Thomas and Mack Center
http://www.findlasvegaseventsbuytickets.com/search/label/Thomas%20and%20Mack%20Center%20tickets

Have fun and Happy Birthday!
References :

Go see the Blue Man Group.
Go see the Fab Four.
Go to the Shark Reef at the Mandalay Bay.
Go ride the rollercoaster at the New York New York.
Go to the Stratosphere.
Go to the restaurant in the Eiffel Tower replica.
Go to the mall.
Go see the fountains at the Bellagio.
Go see the inside of the Venetian (the ceiling is awesome).
Go see Fremont Street.

Check out http://www.vegas.com
They give tons of shows and sights to see, plus they have a whole listing for things that are free. :]
References :
I went to Vegas recently. :]