The famous European honeymoon cities have a problem: in season they're overrun. Bruges in summer is a stag-do hellscape. Venice in July is shoulder-to-shoulder. Dubrovnik between May and October is a cruise-ship parking lot.
The fix isn't a different city. It's the same city in November. Here's the list of where that's true and where it isn't.
Transformative off-season
Bruges, late October to early March. The day-trippers vanish. The canals are quiet by 6pm. The chocolate shops are still open and you can stand in front of the Madonna and Child without queuing. Binnenstad.
Venice, mid-November to mid-March (excluding Carnival). The fog is the experience. Cannaregio empties of cruise traffic. Restaurants take reservations again. Acqua alta floods your boots in October-November but the rest of the off-season is dry and dramatic. Cannaregio.
Mostar, October to April. The day-trippers leave by 6pm in any season, but the off-season multiplies that effect. The bridge at dawn with no other tourists is the trip. Stari Grad.
Dubrovnik, November to early April. Same logic — the cruise ships dock from May to October. November Dubrovnik is empty walls, your own afternoon view from Mt. Srđ. Old Town.
Cinque Terre, late October to March. The villages become real towns again. Shopkeepers have time to talk. Some restaurants close but enough stay open. The hiking trails are quiet.
Český Krumlov, October to April. Day-tripper density peaks May-September, drops 80% off-season. The town becomes itself again.
Year-round more or less the same
Paris. Crowded year-round; the seasons matter for weather more than density. February is genuinely quiet but cold and damp; June is busy but pleasant.
Rome. Same — overrun in any season the average tourist would consider going. November rain is the off-season trade.
London. Year-round busy. Christmas decorations are nice; April daffodils are nice; otherwise pick on weather.
Amsterdam. Same — if anything more crowded in November (truffle season, less queue management at the Anne Frank House but more concentrated tourists in the few months without rain).
Off-season makes them worse
Anywhere on the Greek islands. Most ferries reduce to weekly. Most restaurants close. The villages can be 70% shuttered in February.
Coastal Croatia, Albania, Montenegro. Same problem. Beautiful in shoulder season; basically closed in deep off-season.
Mediterranean beach towns generally. The rule: cities transform off-season; resorts close.
The shoulder seasons are usually the answer
For most of these cities, the perfect window is late September to mid-October or April to mid-May. Mostly empty of summer tourists, weather still pleasant, restaurants open. November is the dramatic-empty window if you're willing to take rain and cold.
For winter Europe specifically, see the January guide and November guide.
The romance-trip-with-no-crowds itinerary: pick one of the above transformative cities, go in early November, book a hotel that has a fireplace or canal-view, accept that some restaurants will be closed, and have one of the better trips of your life.