In the book Hamming tasks the reader with creating in some detail a vision of your future. I did do this, but lacked the foresight to effectively find a way that I could shape my work time to be "Hamming" meaningful.
As an example from the book, Hamming tells a story in which he sits with a group of chemists at lunch. He then challenges them to come up with the most important problems in the field. They did so, and none of them were working on those problems. He ended up challenging them to explain why they had not sought to work on those problems, and was no longer invited to the lunch table.
In this case, Hamming's idea of a meaningful career is a really singular one: contribute to solving the most important questions in your given field. This worked out very well for him, and he claims to be an ordinary man who worked hard over a long period of time. For myself, I'm not sure what the important questions are quite yet :)