I think you can get away with two days in Nice if you are somewhat frugal with your time. We spent about a day and half in Nice and were able to walk all around and make a side trip to Monaco.

Unfortunately, we only made a side trip to Cinque Terra while traveling between Nice and Florence, leaving our bags in La Spezia (I think). Ideally, I think we would have liked to have spent two and half days walking through and between the towns.

But if you have time to spare, I'm sure you could enjoy just lounging on the beach in Nice or taking it easy in Cinque Terra.

[No experience with Genoa]