Monaco is beautiful and interesting; we did a walking tour of Monte Carlo and a drive down the coast. Cannes is a lovely and expensive beach town. Given a choice, i would see Monaco.
It would be a shame to go to a facinating and beautiful place like Malta and just go to a beach, although they are lovely. Malta is one place I would suggest taking a guided tour from the ship. Malta has a rich culture and history and a good tour guide can bring that all to life for you. And Malta is small enough that you can see it all in a day and still have some free time for shopping and maybe a beach.