Free, quick, & readily available! With two tracks per stop, we never waited longer than 3 minutes for a tram to come. They have digital displays on each side telling you how many minutes away the tram is. The tram travels between Monte Carlo (w/ access to Aria), Crystals shopping center (w/ access to Aria), & the Bellagio.
You do have to walk through some windy long hallways in the hotels to get to the stops, but it is still much better than navigating the busy streets in the hot sun!