La coupole is a good one, Auschwitz you have to book a tour in advance. Mimeyeques, Tyne Cot, are good as well. Never been to Colditz.
Stalagluft 3 (great escape) is worth a visit.
If you want to see a lot in a short period just follow the western front (WW1) or go to Normandy (WW2)
Done a lot of both, if I can help give me a shout