Monaco is easily accessible from Nice - about 1/2h by train. We spent an enjoyable afternoon and evening there, visiting the cactus "exotic" garden, castle, casino and cave. The cave was particularly interesting - it was about 18degC inside (despite winter temperatures outside), and walkways for visitors had been made using explosives and diamond drills! Despite the damage, it was still a lovely cave. Monaco was clean and tidy, and felt almost like a movie set.