My 21-year-old son is studying in Madrid this summer for two months, and is eager to do a quick weekend trip somewhere else in Europe during his break (basically 5 days at the end of June). He's seen a bunch of Spain already (Barcelona, Toledo, Valencia, etc.) so is eager to head to a new country with friends. Money is an object (e.g., they are hoping to find reasonably priced accommodations - hostels, etc., will fly a discount airline, etc.).
He's already visited (with his family) Paris, Nice, London, Rome, Venice, Naples, Lisbon so he would ideally go someplace different (although he's not averse to returning to one of those places since he'll be with friends this time).
I know so many people on this forum are expert travelers, so I'd appreciate any advice you have. Thanks!