Night time is to be preferred, especially this time of year as the temps in daytime can exceed 115F. Of course, shooting in daytime can be done if you pop in and out of the hotels/casinos. Do know that you can usually bring even a pro DSLR into a casino but don't be shooting pictures of people in the place or action at the tables and most deffinately not the security/money folks.
By night you have ...
The fountains at Bellagio.
The pirate ship fight at Pirates of the Caribean
Numerous street shots of the buildings from street level and from the walkways over the street
My best guess is that you will likely see someone that looks, or tries to look, like Elvis and more than likely one or more of the pretty girls won't be -- a girl.
Of course if you do venture away from LV there is the Valley of Fire about 65 miles east and Hoover Dam about 30 miles south.