We were in Vegas recently for 5 days -
Highly recommend the Mob Museum tour, add in a couple of experiences in the museum too (speak easy to try their moonshine and we did a forensics experience), really enjoyable.
Night time viewing of the Bellagio fountain and Bellagio's has a botanical garden inside thats quite spectacular
Grand Canyon West (don't bother with the skywalk, que for hours to walk on a glass balcony) and Hoover dam (can cover both in a day comfortably)
Re-do wedding vows with Elvis
The Mob Museum Slogan was fantastic "There's two sides to every story, and then there's the truth!"
The Bellagio breakfast buffet has to be seen to be believed.
Another great excursion is to Zion National Park. There are many awesome natural areas within striking distance of Vegas. That whole SW region is amazing.
When I was last there, Mandalay Bay had an amazing aquarium exhibit, including sawfish which are quite rare and difficult to see in the wild.
If you're in Vegas, it's a good idea to check out the stands selling discount concessions. You can get great deals.