Learn to ballroom dance
Read / research various things
Exercise (learning a martial art is best)
Draw/Sculpt/Create something
Learn to cook well
Learn a new musical instrument / Create a band
Learn a new sport
Perform in a play
Develope a night life
Organize or attend a group / club
Get a girlfriend/boyfriend or spend more time with her/him
Create a website
...


The best thing to do is to try and learn something that interests you. Being productive has a very lovely feeling.