I recently wrote Getting Started in Tinkercad (PDF), a free project, ready for you to download and use. If you have kids home from camp, this is a great activity to keep their brains firing till school starts!
The project uses my usual-step-step method to work through this cute little boat:
The Tinkercad team has a similar boat printed out (I believe from a MakerBot):
Anyone can design anything in 3D! http://www.3dvinci.net/