For anyone who is 18 and has a drivers license and health and auto insurance, I HIGHLY recommend you go to the Sahara and do
GM The Drive. For $10 you will get four laps around a track in a new Corvette. Or if you prefer, a Cadillac, Hummer (off road course) or any of many other vehicles. I'd call it one of the best entertainment values in Vegas.
For those under 18, there are Circus Circus carnival games (the ones you think you can win but won't then end up spending more money on than the prize is worth), but that place has kinda gone downhill in recent years. Excalibur has a similar thing that is a little better. The Caesar's Palace Forum Shops are pretty neat, as are the ones in the Venetian, and inside New York New York. There is also the wax museum at the Venetian. There is a pretty good coaster around New York New York, but it is a little scary (not thrilling scary but "is this thing built and maintained well?" scary). They close it down if wind gusts get over like 30mph. That tells you something. I've ridden it once. I don't think I would again.
For shows, the Pirates show at Treasure island is free but isn't so family friendly after they redid it a couple years ago. The Mirage volcano is also free to watch and is pretty neat, as is the Bellagio water show.
The main thing about Vegas is that like EVERYONE there smokes (how else would you pass the time waiting for the slot to win huh? j/k) So be prepared for that.