As it's a cooking forum, I'll restrict my response to food-related stuffs. Predominately because there is no "must do" thing in Vegas--what you do is up to personal tastes in entertainment, and there's a bit of everything.
Best restaurant in Vegas: Rosemary's. HANDS DOWN. You have a moral obligation to eat at Rosemary's. Pay no attention to the fact it's located in a strip mall. Pay no attention to the signage. Go inside. Eat. You must. Don't ask me what's good. Everything is good. If it seems like your food is taking a long time, it's because they're re-doing it because they have very exacting standards. Stuff another honey butter scone in your mouth and smile. Do it.
.....you're also going to have to rent a car, cause it's WAY off strip on Sahara. It's a locals secret
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As for on-strip dining:
-Italian: Rao's in Caesar's
-French: Andre's (1 block off strip)
-Fusion: Roy's. Not technically on strip, but not too far off it. It's on Flamingo
-Asian: Pearl in the MGM
-Steak: craftsteak in the MGM
-Mexican: Diego's in the MGM (seeing a theme here? The MGM as a hotel/resort/casino sucks, but they have the best restaurants)
-General fine dining: Joél Robuchon in the MGM
-Food on a stick! (initially)--Texas De Brazil Churrascaria. If you've never eaten at a Brazillian steakhouse, you simply must go here. And yes, it fulfills the "all you can eat" requirement people seem to have about Vegas.
-Prime rib: Lawry's. Again, not on strip but right near it on Flamingo
-Seafood: Emeril's Fish House in the MGM. I'm also told his Delmonico in the Venetian is a wonderful place to eat, but I've never found the time to go there (which is odd since I'm there 6 times a year, usually)
-Burger: Burger Bar, Mandalay Bay
And....if you must....if you just can't resist and HAVE to do a buffet: Do the one at the Rio. I won't recommend it because I'm anti-buffet, but it's the best of what's there.