Having a bit of running withdrawal, but I've found a walking routine on the treadmill that works for me on days I don't ride. We've had 2-3 nasty rainy days in a row when even I won't walk outside.
Basically, I am doing 2-3 hill intervals, at a 4-6% grade at 4.2 mph, for 10-20 minutes, interspersed with flat walking. When the 10 or 20 minutes is over, I alternate with the 3 sets of functional weight training on the ball, from the cyclo-core program.
So, I get close to an hour, between warming up for 5-10, 3 sets of the weight/body work, and 2-3 sets of the hill walks. If I am short on time, I can cut out one or more parts. I'm noticing that this is keeping the weight off more than riding... just doing this 2 days a week.