If you do not mind getting wet, then rainy days present an opportunity for some really atmospheric photos. Wet pavements and roads reflecting street lamps and shop window lighting. People hurrying hunkered down against the rain. Lighting that will be dark, but not night. Give it a try.
Yes, you risk your camera getting wet, but stick on a neutral density filter to protect the lens and evereything else should be fine.
And if you really don’t fancy actually get wet, then wait until the rain stops then get out before the streets and roof tops get dry.