Mathematics
Although I didn't study mathematics past college, it's always been something of a nerdy secret interest. While it's true that math occasionally crosses paths with my work (software development), the really interesting stuff – the pure maths – lives in an entirely different, but equally abstract world.
A couple of years before the kids entered our lives I entertained the idea of doing an Open University degree in pure mathematics – which would probably have meant re-treading the same qualifications I already have, because it's been 20 years (yes, really) – I've forgotten almost as much as I once knew.
Part of me wonders about doing the degree when the kids have grown up, but another part of me asks what the point would be – why chase an interest that has no practical use in my daily life? It's not like it's a small hill to climb either – it's a sodding great mountain.