(Because apparently this is the only way to respond to replies right now):
Cork, Dublin and Brussels are out because I’d like some sort of guarantee of sunshine.
I knew I’d forget to mention places I’d already been, and Reykjavik is one of them (as is Helsinki).
A few people have recommended Prague and also Budapest so I’m going to investigate them. Bologna isn’t a city I’ve ever considered but I can look into it – although I don’t drive so I really want to be somewhere I can rely on my feet or public transport.
Thanks for your recommendations everyone!