If you go to Vegas, you have to see Madame Tussauds at least once. I had never been to a wax museum before, and it was a weird experience looking at something that looks SO real yet I knew it wasn't. Once I got over the mild creepiness of it all I had a lot of fun taking pictures of practically everything. I also got a chance to nerd out and get my picture with Patrick Stewart.
The tickets are a little pricey, but you can find coupons practically anywhere.