I actually don't mind a theme park day in the rain... Usually disperses the crowds and queues.
I remember going to Thorpe Park on a truly abysmal cold wet day in June 2002 (started off OK but the heavens opened , sub 15C max, proper October / November vibes).
Most people rushed off home but we stayed ...
I just walked straight onto Colossus, Vortex, Loggers Leap etc again and again , actually ended up getting bored of them ! Better than standing there queueing for hours and getting to go on 3 rides on a sweltering sunny day