When the weather changes, it's worth changing your expectations for the day too. Instead of moving your whole itinerary indoors, choose one main activity and build the rest around it. These ideas are for reorganizing a day of ordinary rain; a weather alert means checking local guidance before you go out.
An exhibition that interests you
CCBB Rio de Janeiro publishes its program on its official website. Start there: see which offering matches your dates and what type of admission applies. Having an indoor plan doesn't remove the need to check availability.
If you find more than one activity, prioritize the one you really want to see. A cultural outing works better when you have time to linger than when you treat it as a checklist to complete.
Another option: Museu do Amanhã
Check the museum's visitor section to plan your entry and see how to get there. Keep in mind that an indoor destination may still involve an outdoor walk from public transportation. Check that stretch and avoid adding a second booking too close to the first.
A break is part of the plan too
You can look up what Confeitaria Colombo offers and confirm the location you're interested in. Before making a detour, check its opening hours and work out whether the trip makes sense from where you are. There's no need to cross the city just to fill time between two activities.
What to sort out before you go out
Save your ticket, confirm the address, check the route there and back, and bring what you need for the outdoor stretch. If conditions get worse, simplify the day. One well-organized visit is usually more enjoyable than chasing three bookings in the rain.
Sources: CCBB program ↗, Museu do Amanhã visits ↗, Confeitaria Colombo ↗. The suggested combinations depend on dates and availability.