City Center is beautiful. It's more modern, not themed like the other hotels on the Strip, but after you get over that, it's quite impressive. It's especially beautiful at night when you can see the fountain built by WET in all its colored glory. It's still pretty amazing during the day too.
I loved the galleries - the glass art by Chihuly (the guy who did the glass art in the lobby of Bellagio), the Cirque de Soleil statues, and the photographs by rLo (Rodney Lough Jr).
The Crystals Shops are real nice too. Everything there reminds you that this is a environmentally-conscious and sustainable place - from the trash cans with the nature designs carved on the sides to the wooden planks lining the stairs. WET has a few more cool features inside Crystals: Halo, which are tubes of water with tornadoes in them, and Glacia, poles of ice melting into cool designs. Both are nicely lit up with changing colored lights.
Finally, if you want to see the Silver River by Maya Lin (of Vietnam Memorial fame), it's behind the registration desk at Aria right when you enter to the right. Huge 84-foot reclaimed silver depiction of the Colorado River.