When the Riviera Hotel opened in Palm Springs in 1958, the area was already an established retreat for Hollywood’s elite—Palm Springs first began attracting high-profile vacationers in the 1920s. The communal-style property quickly became the go-to getaway during the swinging ’60s, and not surprisingly, it established itself as a staple hangout for Frank Sinatra, Dean Martin, and the rest of the Rat Pack. Sinatra continued to visit the property through the 1970s and ’80s, though by that time the hotel had evol…