How to structure a day without ruining it
Anchor the morning. Choose one thing that has to happen before lunch, ideally something that benefits from an early start: a museum, a viewpoint, a market, a hike. Book it if booking exists.
Leave the afternoon soft. This is where the trip usually happens. If it is hot, this is also when you should be indoors or asleep, which is what people who live in those cities do.
Decide dinner in advance only when it matters. One reserved dinner per city is usually the right number. The rest can be whatever looks good on the walk home.
And keep a wet weather option per destination. Not a plan, just a name written down somewhere, so that a bad forecast does not cost you a day of deciding.