Rouen is not very well served by buses, although there's a useful, but very slow, service to Le Havre (€2, three hours, four to five daily) from the bus station.
Most car-rental companies have desks in the train station complex.
Caen €28, 1¾ hours, five or six daily
Dieppe €12, 45 minutes, 10 to 16 daily Monday to Saturday, seven Sunday
Le Havre €16, one hour, 16 to 20 daily Monday to Saturday, 10 Sunday
Paris Gare St-Lazare €10 to €24.10, 1¼ to 1½ hours, 25 daily Monday to Friday, 13 to 18 Saturday and Sunday