On a weekend, one bad day is half the trip, so this is the contingency worth rehearsing.
First, check what kind of bad it is. From June to October the usual event is a violent thirty to forty minute afternoon thunderstorm, not a day of rain. That does not need a new plan, it needs a two-hour indoor block and the knowledge that the light afterwards will be the best of the weekend. A genuine all-day system, which is more of a winter front or a distant tropical system thing, is different.
What actually gets cancelled. Boats, for lightning rather than rain. Open-air rooftops. Outdoor markets, sometimes. Nothing else. Museums, galleries, covered shopping streets, kitchens and the free Metromover carry on exactly as normal.
The straight swap. Move the 9:30am water block to Sunday morning if your flight is late enough, or to the following weekend's credit if the operator offers it — a morning booking that gets weathered is far more likely to be rebookable than an afternoon one. Then run the rest of Saturday in reverse: indoors in the middle, the neighbourhood block at four when it clears, dinner as planned.
The September and October caveat. These are the peak weeks of hurricane season, and while most weekends in them are entirely unaffected, a system several hundred miles offshore can cancel every charter departure under a clear sky. That is the single strongest argument for a refundable rate and for putting the thing you came for on Saturday morning rather than Sunday.