Start with the thing most rainy-day pages will not tell you: South Beach has two museums. That is the indoor inventory. The Wolfsonian-FIU on Washington Avenue and the Bass at Collins Park, and after that you are into lobbies, shops, a cinema and lunch.
This is not a failure of the island; it is what a twenty-three block barrier island with a public beach as its main asset looks like. It does, however, mean that the useful question here is not what to see instead. It is how long you actually have to wait.
In summer, usually between thirty and sixty minutes. From roughly May to October the pattern is reliable enough to plan around: heat builds through the morning, thunderstorms form inland over the Everglades, and the line drifts east to reach the coast in the late afternoon. It arrives fast, it is loud, it can be genuinely violent, and then it clears and the evening is lovely. This is not the all-day grey that northern visitors mean by rain. If you plan the beach and the walking for the morning, you will rarely lose a day.
In winter, occasionally a whole day. The rarer version comes with a cold front in December through March: a grey, breezy, drizzling day in the 60s that sits over the island and does not move. There are only a handful of these a year, but they are the days when knowing the indoor list matters.
And the island's size turns out to be the advantage. In most beach destinations, rain means being stuck wherever you drove to. Here you are never more than fifteen minutes' walk or one free trolley ride from your room, so the correct response to a summer storm is very often to go back, wait it out and come out again at five when the light is beautiful and the pavement is steaming.
Two local hazards come with the wet rather than after it: lightning, which closes the beach and is not a formality, and king tide flooding on the low western streets in September and October, which happens on clear days as well as wet ones.
What follows is the honest indoor list, the things that still work in the wet, and the point at which crossing the bay becomes the right answer.