Hi Kirstin!
The best recommendation I can give is to go to the theme parks! While most guests probably do not want it to rain on their vacation, my experience is the rain thins out the crowds at the theme parks, especially if it is more than just a passing storm. Most of Walt Disney World can be enjoyed in the rain! Rain ponchos are available throughout the theme parks
It's easy to visit the Magic Kingdom and Disney's Hollywood Studios as the attractions are fairly close together, making it possible to dart from one attraction to another. It's somewhat easy to visit Epcot in the rain as the Pavilions are generally large and offer a lot of things to experience and see; if it is a fast moving storm, you might be able to stay inside one Pavilion the entire time. However, since the park is large, it can be hard to get from one Pavilion to another.
The only park I would recommend not visiting in the rain is Disney's Animal Kingdom. Most of the attractions are outdoors and there's only a couple of places to "hide" from the rain in my opinion.
You can also visit Downtown Disney, especially the
World of Disney. The World of Disney is the largest Disney store on Earth. You can easily spend a lot of time there; however, it can be packed during the rain, so it may not be the best choice for a 21 month old.
Have fun planning and write back if you have any more questions!