Do not replace the destination — replace the brief
The most useful alternatives are based on what you wanted from the original city. If you were drawn to Barcelona for food, architecture, neighbourhood energy and the sea, Porto offers a different combination with historic streets, a riverside setting and a strong culinary identity. The point is not that one is “better”; the point is that your travel brief might now be better served by a different place.
Amsterdam → Gdańsk: waterfront character with a different rhythm
Gdańsk offers a historic centre, waterfront atmosphere, colourful architecture and a very different scale from Amsterdam. It can work well for travellers who love walkable historic settings and water but want to discover a city that feels less familiar.
Venice → Ljubljana: rivers, bridges and a smaller city rhythm
Ljubljana is compact, with a historic centre shaped around the Ljubljanica River, bridges and public spaces. It is not a replacement for Venice — and it should not try to be. Its strength is that it gives travellers a beautiful riverside old town and access to nature beyond the city without the same scale of tourism.
Vienna → Bratislava: a useful pairing, not a competition
Vienna is famous for imperial architecture, museums, cafés and grand urban spaces. Bratislava has its own historic centre, Danube setting and Central European character. Rather than presenting one as a cheap substitute for the other, think about the pairing: a larger cultural city followed by a more compact chapter.
The travel-design trick: keep one familiar element
If you change the city, keep something else familiar. Keep the same kind of hotel atmosphere. Keep food as a priority. Keep the waterfront. Keep the old-town walks. By preserving the element you loved, the alternative feels intentional rather than random.