Vegas is definitely a place for individuals over 21, so your hedonism is limited. There's still plenty of things to do, but there's little chance to no chance you will be able to sneak drinks, play slots, or go to clubs if you were so inclined.
When I was there last year, I went on a tour of Death Valley. Being in the middle of nowhere is completely different from the hustle of the city. They may not be doing tours this early in the year due to heat, however. I went in late October, and it was still 95˚there.
Shelby American was alright. They have some awesome and rare cars on display. I found that it got a little monotonous after a while, but it is good for killing an hour or so.
I understand it may not be convenient or possible, but you could wait another month to go. You will be 21 then, and SEMA will be going on.