The World Series of Poker cash game room at the Rio isn't the most thrilling for ambiance, but it has the widest selection currently available, and you can also peek in on some of the biggest cash games in the world (tho' please don't linger). The Amazon tournament room, however, has that big blue room final table stage you see on TV, and it's absolutely worth checking out.
Aria has a very nice casino and poker room. I'm also partial to Golden Nugget downtown, but that might be difficult for you to get to easily, if you're on a time budget.
You must check out the WSOP at Rio, you'd be crazy not to.