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have you exhausted all the possibilities of iTunes, iPhoto, Imovie, iDVD, iChat all the free games etc.etc.
Then of course there's non-Apple offerings
 
I have found Chess on my MacPro and on my MacBook is "Big Bang Games" which consists of Checkers, Tic-Tack Toe and about 6 others. Am I just lucky?
 
What are some free applications or fun things to do on a mac

Edit some images, still or moving. Build a show with them. Make some music to go with the show in Garage band.

If you need a subject try "washing a car" in multiple genres like "how to", "music video", "covered as if it were a news event", "in contrast with something else"...

If you are really bored try and organize a classical or jazz music collection in iTunes. That should give you something to think about for months
 
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