Trying to save the world!
It's an SI that I've had since I was tiny and obsessed with nature, living things and life in general. Now that I'm older, I'm able to actually DO something about it. I'm trying to downsize, green my home, work out if it's practical / possible to get microgeneration working at home, working on knocking down the garage so I can have some space to grow veggies and live the "Good Life".
Unfortunately, I do work in admin (OCD makes filing fun! and never have time to really DO as much as I'd like. Twice as unfortunate, it means I'm often browsing "save the world" type websites (or others relating to any other SIs) during working hours, so I'm always paranoid that work will check my web-usage and fire me! Oops!
Saving the world has been an SI since I was little, but I've gone through:
Buffy tVS (for several years - to the point I spent £'000s on autographs and posters and "stuff")
World of Warcraft (I try to keep this to "hobby" level of obsession, but every so often it takes over)
Animals - especially Cats (another lifelong SI, but I am lucky enough to have 4 kitties of my own now
AS - Started looking into this when my partner was diagnosed, to find that I'm most likely quite high on the spectrum too and haven't stopped being interested since I found out there was a "diagnosis" to put to my personality quirks and oddities. Still haven't got an official diagnosis as a) I'm female and b) when I was younger, my little sister would tell me off for not being "normal" and I loved her SO much that I'd seriously repress who I was so she wasn't upset at me or embarrased by me, that I can come across as normal even if I'm killing myself inside to do it so I'm not sure they'd take me seriously.
Famous 5 and anything by Enid Blyton for that age group (again, when I was younger)
Programming for the Acorn Electron (still young)
Snooker players (I didn't like snooker, but was obsessed with memorising their names when I was very young at about the age of 4/5 I could list over 50! - I'd say this was my first SI that I can remember that stood other than the "save the world" thing - I was telling people to switch off lights before I really knew about CO2 or HOW/WHY switching lights off helped - I just knew that it DID!)