What to do over the summer??

I am a current high school junior, and I will be applying to Columbia for Early Decision. I just took the Michigan statewide ACT in March and got a 29, and I realise that my score is not exactly on par to get in, and my 3.7 may hold me back, but as long as I get good grades in the next two trimesters of my career; I will be able to apply with a 3.8. Besides the obvious fact of boosting my GPA and ACT, what could I do to boost my extracurriculars? I live in a tiny town in the “tip of the mitten” of Michigan and basically only have NHS as far as clubs go.