I never get a car when I'm in Las Vegas (I go about once a year) so you should be fine on that front. There's plenty to keep teens busy, but unfortunately most of it costs money. There are lots of arcades and such, but they're going to cost ya
One place to get info on LV is:
http://www.vegasfreedom.com/do/index.jsp
Here you can select the dates you'll be in town and see who/what will be performing while you're there.
Another resource I really recommend is
http://billhere.com/
If you sign up for the newsletter you will get updates on what's opening/closing (he usually has the concert info before the official website I listed prior). Also, the coupon service is a great deal -- lots of 2 for 1 offers that should save money. Basically they're taken from the free magazines you can find around town (at the Mirage you'll have one in your room) and just mailed to you so you have them in your hands before you leave town.
Gameworks is a place that usually has some kind of "pay for $10 and get $20 worth of play" coupons -- that'll help entertain the kids. If you go through the coupon list you should get a lot of ideas -- kids or no kids.
alt.vacation.las-vegas (in google groups) has a wealth of information. It's currently overrun with political nonsense, but if you scan for trip reports you'll find lots of valuable insight from travelers.
And for the person staying at the Mirage, there's a machine (or was, in July) in the area leading to the Siegfried and Roy theatre that has a machine called "Rich Family" or something which features... A BOXER! It's brindle and running with its tongue out. I played for a while, hoping it was a sign of impending gambling luck, but no luck! I think it was a nickel game
I took a guided Hoover dam tour this summer and it was about $30. That included pickup at the hotel and transportation to the dam (with chatter along the way about the history of the area we were passing, including celebrity homes). We were dropped at the dam -- you can just walk around yourself, or take the tour that goes inside. I forget how much it was $10-$15 additional, I think. Can't say I really recommend it. It's not the "dam tour" that the family takes in Vegas Vacation -- very short. Still, I figured I was there, so... On the way home we stopped at the Ethel M Chocolate Factory for a free tour and samples
To find a good buy, just do websearches. You'll find tour groups and compare prices. Most were in the $28-35 range for the exact same tour. I think I got the lowest rate for booking through the internet.
Edit: Note, it also included a stop at a casino way out in the boonies for lunch. The casino was nice enough -- the buffet lunch was GODAWFUL. So don't bring a big appetite. It was tell-your-grandchildren bad. That was a 6 hour tour overall, I think. They have 8 hour tours, too. For Hoover Dam I'd take the shortest one
