I haven't been there for a while, but...

There's some nice solid stone wall around the fountain/pool at the Bellagio, to set a camera on if needed.

For accurate exposure prediction, visit flickr and look at the exif data of other people's photos. With portra you need not worry about the brights blowing out, just exposure for the scene. Multiple exposures if possible without moving the camera will also make a nice water texture.

At the Bellagio, I had more fun people watching than fountain watching actually. People came in to view the fountain from everywhere, and people were constantly streaming in and out of the place for gambling/lodging as well.

Visit the stratosphere as well.