Rome. The romans left their stamp all over europe. I have been to Rome during all seasons and yes, it gets hot, but it's not the damp kind of hotness that everyone loathes. I love the city and I highly recommend to NOT just run from tourist place to tourist place.
Learning a few basic frases in italian will also earn you many smiles and people who will try to help, even if they do not speak a single word in english. My cousin lives there.