Ok I know this may sound crazy because I mostly believe in doing the exercise that you are training for in order to do that exercise.
Have you heard of kettlebells?
You can buy them at Target. Anyway, one book I was reading was talking about athletes training with kettlebells (no running) versus those who ran as part of their training. When compared side by side at the end of a training period. those who trained using kettlebells actually did better than those who trained by running. They are a great indoor activity.
As for outdoors, I'd recommend getting yourself some yaktrax if you don't have them already. You can buy them at most sporting good stores.