When we decided to home school, I asked Catherine what she did at her school that she wanted to do at home also. One of the few things she told me was "bottle cap art." Her school made huge, elaborate pieces of art work out of plastic bottle caps which the entire school had collected for months. We of course could not do something on the same scale, but we did a modified version.
We collected plastic bottle caps for several months. Then we washed and sorted them: