I'd say Celle and Bremen. Bremen is a very nice city and there are some things to see like the EADS space centre and Becks brewery. Celle is a cute little timber town (not sure if it's real or fake).
Personally I don't like to go so far by train because if you have to change train it takes time. Regional trains are slow too. I do think Bremen and Hannover has good train connection, and Celle is not far from Hannover so it should be just a short train ride and you are there.