It depends on what your kids are into honestly? For example, we do a sumo camp with our kids during the off season. Also the kids do many demos and work on different projects and getting the younger ones up to speed.
Making it to your championship event then world championships is also a great way to kill time.
Learn more about how the Ev3 and NXT are resourced and all of the wonderful things other teams have put together with my blocks. Learn new types of mechanisms with the NXT. I have heard of teams do entire games with 2 motors
Another thing you could do is have your teams research a bit more on what other projects people talk about at competition. Ie have them learn about tsunami's if they did tornados.
Lots of stuff can be done. it's all about what you and your kids would like to do?