One thing I have wanted to do for years is a "rain window". It's a regular window sash complete with curtains mounted on, say, a 6" deep or deeper frame. The inside back would be painted or covered with a typical outdoor scene. The inside would be water proof. A controller of some sort (RPi/Arduino/etc) would control a fan, water pump or 2 with some spray heads of some sort, maybe some valves, sky lighting and a strobe light, along with a sound synthesizer to reproduce thunder and wind sounds. Programmed correctly, it would simulate a rain storm from start to finish.
Or, sync the hardware to a CD like this:
If you really wanted to make it more realistic, you could build a false wall a foot and a half or 2 out from another wall, and have a couple of windows and more room for special effects.