If you're in Nice, the port is a must-see. It's the main port in Nice, and it is littered with two…read moretypes of boats: small, colorful sailboats; and megayachts. It is incredibly picturesque, and there is a stunning view of the port from up on Castle Hill. It is worth walking down to the port as well, because it's such a cool harbor.
I recommend doing it the same day you see Castle Hill, because it is right down the road. If you really want a treat, walk an extra half mile past the port and dine at Le Plongeoir. Even drinks there would suffice, given the incredible view. If you do dine there, make a reservation early: they book up months in advance. Whatever your proclivities, however, you should definitely take a stroll around the port!
Happily,
Clayton Cohn