I second the Forum Shops. They're just amazing.
There are a couple outlet malls nearby. The
Fashion Outlets are a little farther away but there are shuttle buses from some of the hotels. I've never been there but it looks like the stores there are a bit more upscale of the two.
The
Las Vegas Outlet Center is at the far south end of the Strip and on the city bus line. I've been there...I liked it.