Obviously, the Star Trek Experience would have been a good suggestion. But consider that I'm a science fiction geek, computer programmer, technologist, those kinds of things. What kind of cool stuff along those lines is there in Vegas?
I have a few suggestions for you:
Go to the BlizzCon 2011 in Anaheim, only 60 km away from LA. There you can take a look at Diablo 3 and you could use collect some fancy gifts from Blizzard.
Go to the Atomic Testing Museum. I was never there, but a friend of mine really recommends it. And according to the homepage it sounds quite interesting.
I don't know what kind of geek you are, but maybe the Erotic Heritage Museum has something for you?
Something I don't know personally, but also sounds interesting: CSI: The Experience
Maybe DEF CON is something for you. It is a hacker conference that takes place every year in Las Vegas.
Always worth a visit is the Playboy Club!
Drive around and see if you can get a glance at Google's self-driving cars.
Contact the Geeks of Nevada and see if they can help you.
I'll add more activities as soon as I can remember all the stuff ;)
Speaking as someone with similar interests as yourself:
1) Best geek thing to do in Vegas: learn to count cards at blackjack, play poker, or other forms of advantage play. Very few gamblers believe there is anything to learn about gambling, and so it's possible for geeks to learn how to maximize their gambling experience. Poker is the easiest game to become good enough to win money. Play blackjack and video poker with a goal of breaking even, but generating casino play so you're offered comps. I win money at poker, lose money in the casino, but my comp value is usually ~3x my casino losses.
2) Been to the Atomic Museum twice. It's definitely worthwhile, but won't take more than a few hours even if you're extremely interested. Use Google Maps to take the bus there.
3) Actually, use Google Maps to take the bus anywhere. Cabs are hellishly expensive, the monorail doesn't go very far, but the bus network is extensive and cheapish. ($7 for an unlimited day pass as of mid-2011.) I almost always walk where I'm going, but not in Las Vegas -- the distances there are much further than they appear to be, thanks to topography and the size of the landmarks.
4) There are half-price show ticket booths scattered around town. Quite a few of the shows would be of geek interest, purely for the live special effects if nothing else.
5) There's a nightclub called "Insert Coin" near the Fremont Street experience which is apparently themed on 8-bit video games. It wasn't open yet when I was there, but the "coming soon" sign was a "Loading" bar that was progressively filled in red. Thought it was a great promo.
6) Helicopter tour of the Grand Canyon has been on my list for years. Haven't done it, but it strikes me as geek double-rainbow.
You will probably have to take a cab there but it's pretty interesting to see all of the old Pinball machines from yesteryear.
You can always go to CES - Consumer Electronics Showcase
This geek enjoyed The Neon Museum for its history and tech aspects. Several pictures I took at the Boneyard are now in rotation as desktop backgrounds.
There are tons of non-casino things to do. For example if you like zoo-like activities, at different hotels you can see lions & tigers, flamingos, dolphins, sharks & rays (and you can touch a ray), assorted fish and birds both inside and out, and so on. And if, like some geeks, you're a foodie, then there are more places to go than you can fit into any vacation - and for less than you might expect to spend, too. I also enjoyed figuring out the tech behind the free outdoor shows like the fountains, volcano, pirate/siren battle and so on.
The bus is a good bargain. The distances are enormous - walking will fool you a few times.