We were in Vegas last week & went to both of the shopping areas Claire L mentioned plus the Primm Outlet Center.
I love the Fashion Show Mall - they have a huge Macy's, a Nordstrom, a Dillard's and a Robinson's May which is similar to a Macy's, plus a lot of 'regular' shops you see in malls, plus some that might be new to you.
The outlet center south of the strip wasn't that great - just ok. They did have a very large
DisneyLAND outlet there which had a ton of stuff at very good prices. It is right next to the Sacks 5th Ave outlet. Also in that center is a Gymboree outlet. There are 2 or 3 different sections of that mall, I drove from the section with the Disneyland outlet to the larger one with all of the rest of the stores. Besides those 2 stores I didn't find much else worth noting, but it depends of course on what you are looking for.
On the north end of the strip were the "premium" outlets, this is an outdoor mall so you might want to go at night rather than in the heat of the day. This one was nicer than the one at the south end and had a very nice Coach outlet.
Another option is the Primm outlet center which is an easy 25-30 minute drive straight down south on I-15. They have a Williams-Sonoma/Pottery Barn outlet which was very nice. That mall is more upscale than the mall on the south end of the strip. They also had an Old Navy outlet (which I've never seen) and a Gap outlet.
Let me know if you have any questions on Vegas - we go a few times every year. Have fun!