I live and breath FIRST even in the offseason. After Championships were over this year I was desperate to find a way to stay immersed in the world of FIRST. (Thats when I joined CD)
Here are some of the ways I do this:
1) Go on CD nearly every day (and try really hard to restrain myself when I have homework to do. CD is very distracting when there is homework to be done

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2) Work on PR stuff for the team. Go to local businesses with other members of the team to try and raise money. Organize a few presentations that we are planning on doing. ex: Rotary Club, Lion's Club, Chamber of Commerce expo. Help write new parent handbook.
3) Attend offseason competitions with my team. They are oasises of sorts...super-fun breaks in the long stretch of time between the end of the season and the kickoff.
4) Work on off-season projects with my team, such as converting our 03 bot into a small ball herder/collector.
5) Teach rookie members of the team how to do stuff.