Cannes

What are your interests? The Riviera offers a lot of options, and the rail line along the coast--while not fast--is convenient and inexpensive.

Are you going to stay in Cannes? It sounds like you are passing on any form of land excursion. You have the Le Soquet (the small old town) right there to explore and the Provencal Market not far away. You can head to La Croisette and shop and dine. I'd recommend making reservations for lunch at a recommended place.

Alternatively, you can board a train (probably 15 to 20 minutes by foot) or bus and go to Antibes or Nice. (Assuming you'll stop in Monaco, as is pretty common -- and won't go to one of these others). Antibes features a lovely old town, the Provencal market, and the Picasso museum. You might be able to squeeze both in, as the train is quite short, if you head in the morning to Antibes and return back to Cannes after lunch. In Nice you have Cours Saleya, the Promenade, Castle Hill, shopping and a number of museums, depending on your tastes (e.g. Chagall, Matisse, modern, etc).

In Cannes we went to the fort above town that gives you a good view over the city--Chateau de la Castre. We walked along the waterfront, and took a look at the film festival building. That was about it. I would recommend taking the train from Cannes to Antibes.