The Leineaue south of Hannover is a large wetland complex. I haven't been myself, but it gets a chapter in
this book
Winter species include: Greater White-fronted, Bean, Greylag, (Barnacle and Pink-footed, both sporadic) geese, Dabbling ducks include Northern Shoveler and Northern Pintail plus the usual suspects, Sea Duck include Velvet Scoter in limited numbers but regular, Goldeneye, Smew, Goosander, divers and grebes, (doesn't say which though), Great Cormorant, 90% of which are
sinensis in Germany, Great Egrets, Hen Harriers and White tailed Eagles.
You might catch the migration still which includes waders at Clauen (mainly) and Lehrte. (Search the fields at Clauen for large flocks of Northen Lapwings and Starlings), Ospreys, Western Marsh Harriers, Golden Plover, Forster's Terns, Common Cranes, and other small birds are all possible. At Clauen there are often sightings of rarities.
I hope this helps. If you want directions for travel, let me know your travel options and I can give you directions from the book.