Photoshop is my all time favorite, but I play with iPhoto and iMovie too. I make custom playlists in iTunes all the time or else I find I listen to the same music over and over. Let's see... podcasts can be pretty fun too...
I've also organized my life with Address Book and iCal. Just in time too since I was able to sync them with my new iPhone. Sounds boring but it feels pretty good to get those little details taken care of in an organized fashion.
Plus... BBEdit for coding... Omnigraffle for flowcharts and algorithms... OmniOutliner for scheduling and budgeting... iWeb to publish quick and pretty web pages so my distant family can watch my kids grow... Onyx to optimize and tune up my system...
Just turn your web browser off for a few minutes and outline a few personal projects you'd like to do. Then put them in some sort of order and get to them one at a time. Start a journal/blog, do amateur page layout with Pages, brainstorm a business plan for your next get rich quick idea, create a podcast (utlize that camera and microphone in your computer!), log on to Skype and meet some complete strangers from eastern Europe. There's a bunch of apps that came with your computer, maybe you can master a couple of them over the weekend... I don't know, I really haven't found boredom to be a big problem on my Mac. And I'm not a gamer but I'm sure there's a few decent games out there for your Mac.
Photoshop is my all time non-Apple app though, I'll bet I spend over 8 hours a week using it.