Are you going to LAS because you want to go there or because it's easy (in the scheme of things) to get F?
If it's because you want to go there then you must already have at least an idea of the things you want to see and do?
The Grand Canyon is a day trip for example.
Plenty of thereads here on things to do in Vegas -
http://www.flyertalk.com/forum/las-vegas-463/