This is a very timely question because it's raining in here right now. What I often do whenever it's raining is to bum out in our apartment because nothing compares to the feeling of being one with your bed. Kidding aside, I usually just stay at home, drink coffee or hot chocolate, and catch up with reading books or watching series/movies.
Rainy days also has this certain effect on my mood, which then drains my energy to socialize. And this just adds to the reason why I prefer staying at home. On another note, since I'm sharing apartment with my closest friends, we sometimes buy some booze and drink to our heart's content.
But the most predictable thing I'd probably do during rainy days is to sleep for as long as I want. After all, the sound of the raindrops as they hit the roof is very inviting.