We had a couple days in Nice on our own, and the most enjoyable experience was a leisurely visit to the Chagall Museum. It has a nice collection of Chagall's work, but it takes no more than an hour to see the collection. What we loved was the film about Chagall's life, which I recall runs for about 45 minutes. It alternates between a showing in French and an English showing, so you need to check out the times as soon as you arrive to allocate your time accordingly. One other pleasant thing to do is to take the (free) elevator to the top of a bluff overlooking the old Nice harbor. It is shown on most tourist maps, and the park at the top of the bluff is beautiful, not to mention the view of the town and the Mediterranean.