Rome
Three thousand years of layers, and the best carbonara within a mile of anywhere.
Better on the second visit, when you stop trying to see everything.
Paris disappoints people who treat it as a checklist and rewards people who treat it as a place to sit down. The museums are as good as advertised, but the trip most people remember is bakery-park-market-wine, repeated in different neighbourhoods for four days.
Pick two or three museums, not six. Stay somewhere with a bakery on the corner. Walk between arrondissements rather than metro-ing between monuments, and the city clicks.
Terrace weather, long light, and the gardens at their best without August's shuttered shops.
A workable shape rather than a schedule to obey — two fixed things a day, and room to change your mind.
Walk more than you plan to. The metro is dense and fast; a Navigo Easy card works better than paper tickets. Vélib bikes are good along the river and terrible in traffic.
Paris is expensive enough that budget differences show up fast. Agree early on how many sit-down dinners versus market picnics you're doing — picnics are genuinely one of the best things about the city, not a compromise.