Unless you want to photograph everything, a second day in Walsrode is usually not necessary. There is a lot to see, but it is a rather compact place, so a full day should do the trick.
Magdeburg still has some pull when it comes to species, Zootierliste should be accurate in that regard and enclosure wise there is a lot to see on making attractive budget enclosures (something UK zoos could learn a lot from
).
Osnabrück and Münster aren't high priority, both are average large German zoos (so pretty good overall), but unless you really want to see the underground zoo in Osnabrück or are a fan of 1970s architecture (Münster!), I would skip them.
Rostock is a different beast and it is a zoo that flies under the radar here. The Darwineum alone is worth the trip, but this is a solid zoo allround with plenty of large naturalistic enclosures. It is one of my favourite German zoos.
Other options would be to visit Rheine (+ Nordhorn) if you are into large naturalistic enclosures (but without rarities). Or alternatively you could do Vogelpark Niendorf and Zoo Schwerin in one day. Niendorf doesn't have the huge rarities anymore it used to have, but if you like birds it is a nice simple bird park. Schwerin is a solid 3 hour zoo with some good enclosures.
For a short trip and some N-American rarities there is the Arche-Noah Zoo in Braunschweig, which is a relatively short trip from Hannover.
