Next summer I will be going through Europe and would like to do a weekend stopover in one of the cities. I'm currently routing through LHR on AA award, but don't have/want to stop there and can add one way segment to somewhere else.

I can entertain myself in pretty much any city, but looking for some suggestions where to go.

My criteria:
1) Anywhere OW flies out of London
2) NOT Reykjavik, Oslo, Amsterdam, Brussels, Paris, Warsaw, Kiev, Dublin, Sofia, Zurich, Rome

Current top candidates are Stockholm, Hamburg, Barcelona, but I'm not set on these 3 choices and will go anywhere fun.