That's quite the broad question, and could be answered with pretty much anything. How about audio production? That's what I do on my 7-year-old iMac. Still does the job quite well.
What are your hobbies/interests? A computer is just a tool to speed things up.
A database of football statistics.
A manual of woodworking techniques.
Your own music or videos.
The great American novel.
A family tree.
A cure for cancer.
A picture of a cat with some amusing text over it.
The engineering principles for a hover-bike.
To paraphrase Barney the dinosaur: it's as productive as your imagination.