Many websites list a bunch of photography projects to get started on on a rainy day. I would argue that staying inside on a rainy day is missing out on a potential photographic goldmine! Stormy weather can bring the most dramatic of conditions, and when the light breaks through it can turn a scene from impressive to awe inspiring.
Light is the most important factor in photography. Without light there is no photograph, and the presence of aesthetically pleasing light is almost as important! Using natural light takes a lot of patience and research, but when you master the art of predicting the light the rewards will shine through in your photography.