Agree with the off-road suggestion, we do a lot of mountain biking at weekends during the winter, it keeps your riding fitness up and is a lot of fun, (especially when you fall off Will, LOL!). Plus you can hone your climbing skills.
I can't ride outside during the weeks in winter when it's really dark and nasty after work so do a lot of sessions on my indoor trainer. Wussy maybe, but then we do get a lot of ice and snow and I'm just not up to tangling with that on two wheels, it's bad enough trying to stay upright on two feet!