I imagine you can also get buses to GSR and the Sparks Nugget, but they might not run all night. 24/7, if you want to visit Peppermill and Atlantis. Other than casino airport shuttles, there are no shuttles around town that I know of.
Most of the rest of the downtown casinos (Harrah's, Cal-Neva and the Little Nugget) are less than a block away on Virginia Street and are easily walkable - though do not walk to Sands (two blocks west) alone at night, as it's sketchy. You can walk between these three without going outside. Eldorado is part of a three block, three casino complex that also includes Silver Legacy and Circus Circus.
