So many places to choose from!
I've been to Paris, Budapest, Prague, Kraków, Barcelona, Romę, Venice, Vienna/Bratislava, Lisbon, New York, Tirana, Munich, Brugge, Malaga, Amsterdam, Palma- all worked well for 3 or 4 night trips.
My favourite by some way was Rome - I've been three times, and could go back there time and time again. But I think it could be too hot in June. Lisbon was very nice in June, good weather when we went last year, but not too hot to walk about. Plenty to see, good restaurants, some lovely little beach towns not too far by train for the day.
Next on my list is Valencia and Tbilisi.