My idea is a just in case box. Be sure that you and your child decorate it together as an activity. Grab slips of paper and write or draw fun activities that don't depend on the weather on the slips and add them into the box or basket. Maybe an impromptu pizza party, indoor art day, or stop by 123child.com and check out their fun ideas. It doesn't mater but whatever adventure you and your child(ren) choose, make it fun. The point of doing this in advance and together does a couple of things. It makes it an activity that creates anticipation and fun for both you and your family and it also gives your young people a warning that they may have to regroup sometime in the future and it gives them an emotional heads up.
When we talk about character building this shows children that they can plan in advance, they can choose activities that make them happy, they can be creative and they can always prepare for the future and not lose out on fun. Parents for us not only does it give us a great teachable moment with our kids but it also prevents melt-downs. Have fun!