We will be based in Avignon for 3 weeks, where I'm renting a car. Planning our numerous day trips, are there any spots that you would particularly recommend we travel to by train instead? eg. Marseille, or some of the smaller towns where parking could be a problem. Merci.
Small tourist towns in Europe tend to have well marked parking lots for tourists who are their bread and butter. We have never had trouble parking in small towns in France or Italy and small towns don't have train service. I'd probably take the train to a large city or when traveling to cities along the Riviera unless you plan to be driving to little villages up in the hills. There is good train service along the coast of the Riviera.
Just be aware that some of the public parking lots in popular destinations have a horizontal bar that admits cars but blocks buses & other tall vehicles. Those are the lots you want. If you park in a bus lot (one that doesn't have the horizontal bar), the bus drivers may be very annoyed at you for taking up a space -- so annoyed that they may block you in and refuse to move until they're fully ready to leave.