The Mandalay Bay complex (includes the Luxor and Excalibur) is the place to be for me. Everything you need under one roof....best sports book in Vegas, great restaurants, fantastic entertainment (be sure to catch Criss Angel if you can get a ticket), outstanding casino. "The Hotel" is posh beyond belief if you can afford it. If you want hookers and strippers it may not be the best place to be but there are thousands of cabs in LV that can take you anywhere you want to go.
Ceasars Palace pool. It's free to get in, you even get given a free towel. It's maybe not as shit hot as some of the other pools, but it's still pretty sweet.
Walk up and down the strip drinking cocktails, wasted off your face... All day long. See how many escort business cards you can pick up off of the irritating "clicky" Mexicans.
Put on an English accent when trying to get with drunk women. That should go for anywhere in the US really, but it especially worked in Vegas.
Depends what you're into. I enjoyed walking around all the hotels on the strip. Plenty of free stuff to see and do. Blue Man Group is a good show to see.
Spearmint Rhino, greatest Strip club in the US. Will you end up paying out the ass... yes, but if you there with money set aside you will have a good night.