What can I do with one day in Cannes?
The coast from Cannes to St. Tropez is among the most scenic drives of the Riviera. You could take a bus there and back. In Cannes itself, this site may give you some ideas
http://www.planetware.com/tourist-attractions-/cannes-f-az-can.htm
Thank you. The bus trip sounds like a very good idea as do the public beaches and old town.
Eat, drink and be merry.
That sounds the best!!