Hello Namak and welcome on the boards ! :D What part of dark Africa are you from ?
Frankly, I don't see what part of Europe deserves to be avoided !
So much to see and do there...
A 2-week trip is fast gone by, and we usually recommend to count a good 2-3 days in major cities, in order to have enough time to see the main attractions. To that, you add travel time in between cities. So for your trip it would gives something like this :
Day 1 --» arrival
Day 2 --» 1st city
Day 3 --» 1st city
Day 4 --» move to 2nd city
Day 5 --» 2nd city
Day 6 --» 2nd city
Day 7 --» move to 3rd city
Day 8 --» 3rd city
Day 9 --» 3rd city
Day 10 --» move to 4th city
Day 11 --» 4th city
Day 12 --» 4th city
Day 13 --» move to Hamburg
Day 14 --» departure
This is of course a rough itinerary. I assume you're landing in Hamburg, and in my opinion, this place could be done with only one full day, unless you're meeting friends there. But basically, 3 to 5 cities will fill your 2 weeks quite nicely.
My first thought was "why be so close to Berlin and not go ?!", it's such a great city to visit ! But then, it would take you far from Amsterdam and Paris. So Marc's suggestion fits your itinerary /trip duration better.
Also to take in consideration : weather (december=cold and in some countries snow), activities/likings (sports, history fans, arts, basic tourist attractions, party, etc), budget (more $ in Scandinavia and less in Poland...).
A loop is the best solution, so what ever the direction you intend to take, make the itinerary so that you don't waste too much time in the train.
A few ideas :
West --» Germany/Netherlands/Belgium/France or
East --» Germany/Poland/Czech Republic or
South --» Germany/Austria/Switzerland or
North --» Germany/Scandinavia