Thinking of the teens:

In Las Vegas, consider the old "downtown Glitter Gulch area" because of having many casinos close to each other, so kids can wander to see different places. (The fine Golden Nugget would be the place to stay.) The main Strip area is certainly flashier, but each casino is designed to keep you trapped in their place and it's tough hot walking to another place. No matter where you stay, do consider going "downtown" for an evening - it's a more international crowd gathering in the street to view the music videos each hour projected on a screen over the street.....and it can be instructive for kids to see the less glamorous side of gambling, er gaming, at the lower-end casinos.

If kids need Las Vegas stuff to take home, take taxi to Gambler's General Store (between strip & downtown) where they sell decks of playing cards from all the casinos all in one place.....these are used-one decks so sold at low price. Fun to come home with decks from lots of different casinos.

For Grand Canyon, perhaps look at the "Skywalk" for the "cool" factor for the teens.....they may have built a hotel there now, too.
www.grandcanyonskywalk.com

For teens that age, I'd imagine intense interest in what other kids their age are doing....so while in San Fran, perhaps a quick trip to Berkeley to see the stores & events near the University of California. It's a quick trip from SF on the BART subway/train.

Hope this is helpful! Enjoy!