Hi all- throwing myself on the goodwill of the community.

I'm currently planning an approximately 10 day trip to Germany this December with my wife and our two boys (6 and 10).

I am constrained by the will of the people as follows:

My Wife: Wants to see the Christmas markets in Cologne and wherever else they may be in the cities we go through. Cologne is a must, though.

My elder son: Is obsessed with tanks and really wants to see the German Tank Museum in Munster (Lower Saxony not Munster in Westphalia). Honestly, so would I.
He had also really wanted to see the u-boat preserved at the German Maritime Museum in Bremerhaven so Bremen seemed like a good midpoint. It turns out the ship exhibitions are closed during winter alas. In any case I'm not closely wedded to Bremen specifically but I will need to take a day trip to the Tank Museum from a nearby city. Staying specifically in Munster seems counterproductive as it appears to be a rather small town (I'm willing to do so if anyone has good things to say about it).

I'm stuck with these dates too, as I have to be elsewhere by the 21st for Christmas celebrations.

Currently the broad strokes are as follows:

10 Dec (Tues)
AM: Land in Frankfurt 0610. Connect to Cologne via train
PM: Check in to hotel. Cologne Christmas markets

11 Dec (Weds)
AM & PM: Tourist stuff in Cologne

12 Dec (Thurs)
AM: Connect to Bremen via train
PM: Wander Bremen old town

13 Dec (Fri)
AM: Rent car, drive to German Tank Museum in Munster
PM: Back to Bremen, check out Christmas markets

14 Dec (Sat)
AM: Connect to Berlin via train
PM: Check into hotel, get orientated

15- 18 Dec (Sun- Weds)
See Things In Berlin (would be glad for suggestions here)

19 Dec (Thurs)
AM Connect to Frankfurt via train
PM Last minute sightseeing Frankfurt

20 Dec (Fri)
AM Fly off from Frankfurt Airport

Would be grateful for any feedback and/or suggestions.