Tee-shirt shooters are cool. My team (3476) made one this last summer using a kitbot frame and compressed air. We brought it to an FLL tournament in Legoland and shot our team tee shirts to the crowd during the awards ceremony. You can see it in the robots section of our website, teamcodeorange.tk .
That was a summer project after our rookie competition, and we really helped show our team to the community and our sponsors that way (we let kids drive it at the Mocrosoft store as well, and became our show time robot along with the minibot!).
Another good idea for the summer, all depending on your resources of course, is to build a drive base of your choice (though probably not swerve or treads for your first year!) with a mentor. I've noticed that this helps teams with programming, electrical, mechanical, and even CAD modeling practice.
Have a great robotics summer
