Rouen

Both Rouen and Clermont-Fe are worth one day.

At Rouen we stayed at the football ground car park and bussed in. No charge. No facilities.
At Clermont - Ferand there is dedicated area to the North, monitored by a local transport official at the nearby ticket office. Charge €15, no facilities. Walk to Ferand then bus to Clermont.

Rouen is a good place for a day and not too big, not sure of the parking now as its a few years since i visited (google and search for sites list a few) watch out for the underpasses, although i have visited a few times and never even thought of going down one but i know a couple who have lost their roofs. i have parked by the river but i think its not possible now. here's a link to a you tube video but not sure if that one is still ok.