Okay, so as a resident of Las Vegas, I'd like to do what I can here. I'd be happy to help with any questions you might have.
The Cirque shows are by far my favorite. Zumanity (at the New York New York) is the more "adult" of the shows, and KA (at the MGM) is absolutely beautiful. LOVE, the Beatles Cirque show (at the Mirage) is amazing! And "O" (at the Bellagio) is incredible - possibly the best Cirque show. As far as price, true they are expensive, but they're worth it. Mystere (at Treasure Island) might be the least expensive of all of the Cirque Shows. The Criss Angel show "Believe" will be at the Luxor. La Reve is a show very much like Cirque du Soleil, but it's over at the Wynn. The Wynn Casino is definitely worth seeing, and if you can, you could catch Spamalot at the Wynn. It's closing next year, so see it while you can.
By October, Bette Midler tickes will be available again at Caesars. She replaced Celine Dion as the headline show at the Colosseum. Cher is coming too, but not until May I think. I hear Bette's show is supposed to be really spectacular.
But there are other shows like Carrottop, and Danny Gans, that are worth seeing. Penn & Teller are great too. I would recommend The Amazing Jonathan. He's a really funny comedian/magician. I believe Terry Fador has a show here too now - he's the ventriloquist who won America's Got Talent last year.
I'm surprised no one mentioned the Freemont Experience. If you're going for an inexpensive, and characteristically "Las Vegas" type experience, head up to Freemont Street. The Stratosphere is also an option - and both are close to each other.
Since you're staying at the MGM, the New York New York is right across the street. There's the huge roller coaster, and one of the most popular Irish Pubs in town. Nine Fine Irishmen is definitely worth going to. Though, if you want to visit an authentic Irish Pub, go to JC Woughlihan's at the JW Marriot.
Definitely bring your walking shoes. By October, the weather will be perfect. No more of these monsoons!!! And it will have cooled off by then. And if you can help it, don't bother with the Monorail. It's kind of a rip off.
If there's anything else I can help with, let me know! :-)