Dieppe coast and the immediate countryside is OK but its nicer, I think, just to the north. St. Valery sur Somme is nice, not many tourists, as is the Somme bay and the Authie valley north west of Doullens. Going west your have charming, but busy Honfleur and Normandy. However that said, to my mind, the best places are the ones you discover for yourself. Booking .com or AirBnB can't go wrong really. I have stayed in St.Valery sur Somme several times, not to be confuse with St.Valery en Caux to the west of Dieppe. If you do stay in St.Valery make sure your accommodation has it own parking. Enjoy, September is a nice time of year to go.