Cross Training
The alternative to riding in the rain or on a trainer is to, god forbid; do something other than ride your bike. No! I don’t mean watch cycling videos and eat popcorn.
There are many different endurance type sports that you can do to keep yourself fit while the weather is poor, including running, swimming, weight lifting, or in-line skating. If you live somewhere that is blessed with snow, or desecrated depending on your viewpoint, you might cross-country ski, snowshoe, or go mountaineering.
If you are like me, you tend to get over-exuberant when you start a new sport, and end up feeling like someone dropped a ton-of-bricks on you. Hence, I recommend that you start out easy and build slowly into the new sport. For example, during the first week of a running program you might only want to do 3 sessions of 15 minutes each. This allows your joints, tendons and muscles to adapt to the new exercise.
Although they can be quite fun, I don’t recommend team sports. It is far easier to get yourself injured when you are in the heat of battle with your friends than when you are running solo. The sport(s) you ultimately choose to do should compliment your riding, augmenting your weaknesses. For instance if you have poor riding strength then you might want to lift weights, whereas if you have limited endurance you might want to cross country ski or swim.