Recreational marijuana is legal, but supposedly there's no place indoors to smoke it. I'm pretty sure the Fountains of Bellagio is the official outdoor location for blazing. Stand there long enough and you might get a contact buzz, especially at night when the fountains are doing their show.
I've seen the fountains many times throughout the years and the shows always look the same to me. The fountains have lost their "wow" factor but if you're a newb, definitely go watch them.
You can pay big bucks to control the fountain too.