I've never seen a fight in Vegas but I did go to Vegas last year for my bachelor party and I also saw the UFC live when they were in DC earlier last year. Both were great experiences.
We stayed in the MGM Grand when I was in Vegas too btw and I wouldn't stay anywhere else if I go back, I absolutely loved the MGM. Are you going with friends or with your wife/gf? Because what you do in Vegas really depends on who you're with and in Vegas you can pretty much do anything you want to do at any time of day or night. It's like a giant playground for adults.
If you're into gaming, the MGM has a really nice casino. I would also highly recommend that you go to the Bellagio and the Venetian and check out their casinos, even if you're not into gaming, you should see them just for the spectacle of it. Also, Mandalay Bay and the Luxor are right around the corner from you, they aren't as prestigious as the other two that I mentioned but they are very, very nice and are on par with the MGM. All of them have amazing restaurants and places to get a drink (try every restaurant you can at MGM, they are all world-class, my favorite was their Mexican restaurant). There's also a food court for quick fast-food type of eating. If you're with a big group of friends, I'd have everyone chip in and rent out one of the cabanas at the pool at MGM and just hang out there and enjoy a nice hot Vegas day by the pool, putting down drinks and looking at all of the nice scenery walking around the pool.
Definitely go down to old Vegas and check out Fremont Street and check out the Golden Nugget and some of the older places that Sinatra and mob used to go to in Vegas. I'm not a big gambler but I did play a lot of blackjack and roulette while in Vegas and had a great time doing it. Just go in knowing that you're doing it for fun and not to actually make a profit.
Also when you check in to the MGM tell them it's your bachelor party or your anniversary and ask if you can get comps and they will most likely give you something all you have to do is ask. When we checked in one of my friends told the guy that it was my bachelor party (which it was) and asked if we could get any comps and the guy gave me an $80 food/beverage credit for my room and didn't ask any questions.
As far as the UFC goes, it will be fun regardless. Like I said, I've never seen one in Vegas but this one is huge so I'm sure it will be amazing. If I were you I'd definitely go to the fan expo, they look like a lot of fun and I actually think most of the time they hold a grappling competition which is cool. Plus you could meet fighters/trainers. Also, I'm not sure if you have to get separate tickets but I'd look into going to the weigh-ins.