I've been a few times, for sight seeing there's a TON of things.
As mentioned, Red Rock Canyon is a must do if you like nature things. Even if you dont its pretty dang impressive. DO make sure you take plenty of water, refreshments if you go. They didnt have the most stocked up vending machines and they were $$ last time I was there.
Red Rock Canyon Interpretive Association | Las Vegas, Nevada
The strip at night is pretty awesome. There is a monorail that connects various places together which is pretty handy if you get tired or need a break from the heat. You can't really just cross the road either, only really at specific spots or via bridges. Plan your walk accordingly or you might find yourself backtracking a good bit.
Las Vegas Monorail Map | VEGAS.com
Hoover Dam is certainly a good one to see. Try to go at dawn or sunset for some amazing views and to beat the heat
If you go that far, you are really right near Historic Rt 66. When we went we made a day trip of it and hauled to the Grand Canyon and back. It was a full day but we had fun with it and honestly spent a pretty good amount of time at the canyon. Then we had a blast ripping back down parts of old Rt66. A day rental is pretty cheap depending on what you get, but places like Hertz do rent their performance line (err convertible C6 anyone
) for a price. Alternatively you could opt for a helicopter ride out to the canyon, sight see from above, do lunch on the floor and fly back in a day.
There's just about anything in the world to do though. Shooting fully auto guns, driving huge tonka toys (real monster construction equipment), off road adventures, dune buggy racing, etc etc
Have fun with it and take lots of pics