How small should a rainy-day backup route be for a weekend trip?

A short-trip planning question about indoor swaps that stay near the original area.

A rainy-day backup route can become too large if every outdoor stop gets replaced by a famous indoor attraction across town. For a weekend trip, I think the backup should stay near the original area: one covered food option, one indoor stop, one place to wait, and one transport fallback. That keeps the day from becoming route repair. When rain changes a Seoul, Tokyo, or Singapore city walk, how small do you keep the backup plan?