A single rainy afternoon is easy. A whole rainy weekend, with nothing booked, is a different problem entirely.
Morning: one activity. Afternoon: one activity. That is the entire plan, and it is enough. Trying to fill every hour produces a schedule nobody follows; naming two anchors a day leaves the rest of the time to sort itself out, which it generally does.
A weekend with no plans and no appointments afterward is the correct time for the activity you normally avoid because of the clean-up — paint, dough, water play in the bath. Save it for exactly this kind of day.
Mention on Saturday evening what is happening on Sunday morning. A weekend with a known shape, even a small one, feels shorter than one that is open-ended in both directions.
Not at all — some children genuinely thrive with nothing planned. This is for the households where no plan turns into a long, difficult day, not a rule for every family.
Alternate whose choice it is by activity, and say so out loud — "this one is your pick, the next one is theirs" — so nobody feels permanently overruled.
Part of our guide to Filling the unstructured days.