The Rainforest Cafe in MGM has a couple of Saltwater aquariums.
As mentioned the Shark Reef in Mandalay Bay.
Some non reef stuff...not sure if you've seen them before or not.
Ice Bar in Mandalay Bay. Everything is made of ice. $30 cover charge.
Pussycat Doll section in the casino at Caesars Palace
Pretty good buffet restaurant called "The Buffet" in the Wynn hotel. All you can eat King Crab and Prime Rib.
M&M store by MGM. There's 4 floors of M&M merchandise.
I think there's a carnival behind Circus Circus. I didn't go to it but saw ads. Looked kinda like The Stampede here in Calgary.
There's a bar in Treasure Island where they did bottle tricks and stuff. Looked pretty cool.
Have fun!!