Lisbon
Tiled hills, cheap seafood, and the best sunset spots in Western Europe.
A beach, a mountain and Gaudí, all reachable on foot from a single apartment.
Barcelona's trick is compression: you can swim in the Mediterranean before lunch, look at some of the strangest architecture ever built in the afternoon, and eat at 10pm like a local. Very few cities put that many different days inside one city limit.
It's also heavily visited, and the city has been openly pushing back on tourism. Book the big sights ahead, stay outside the Gothic Quarter if you want to sleep, and be a decent guest — it makes a real difference to how the place treats you.
Swimmable sea without August prices or August crowds, and evenings warm enough to eat outside.
A workable shape rather than a schedule to obey — two fixed things a day, and room to change your mind.
Walk, then metro. A T-casual multi-trip ticket is shared between people and much cheaper than singles. Bikes work along the coast; taxis are metered and reasonable.
Beach mornings and architecture afternoons keep a mixed group happy without anyone compromising. Spanish dinner hours mean plans naturally split and re-form — worth having the day's shape somewhere everyone can see.