Both of these places can be done in a day as St paul de Vence is mid way between Nice and Cannes.We took bus no.400 from place Garibaldi and stopped at St Paul.
Quoting from http://gofrance.about.com/cs/regions/a/stpaul.htm
Saint Paul de Vence is a charming hilltop fortified village in Provence, filled with art galleries, boutiques and sidewalk cafes. It’s hard to find something ugly about this quaint village. A walk through its winding streets reveals elegant fountains, vine-covered stone walls and statues tucked into nooks in the walls. There are breathtaking views of mountains and sea. Even the ground below is attractive, as the cobblestones are laid into the shapes of flowers.
Best to come early as a lot of tourist usually flock the place around lunch time
After lunch we took the train to Cannes (Cagnes sur mer) to save the time. It’d take longe rif we continue using bus no.400
The train station is close to la Croisette (where Cannes film festival takes place annually)
If it was winter in Monte Carlo when we were there, Cannes is at the mid of summer.
We came back to Nice using the train after dinner