Run 10x 1080p files simultaneously and see if it stutters. Double up if it doesn't. Bonus points if it's pr0nz/hentai at loud volumes.
Folding@home. Because you can
Open a word doc and write 100 lines starting with... "My computer so sexy because...." can substitute 1 line by playing a Rammstein clip from youtube super loud.
Sing, sing a song of sixpence. Wait, that doesn't involve your computer beyond the extent of googling the lyrics. Do it anyways.
Research a lot of grimm's fairy tales. Email friends asking why people watered the stories down. Again, lots of google.
Find the hottest sexiest wallpapers you can find. Create a rotating desktop wallpaper. Bonus points if all pictures do not qualify for PG13.
Google video documentaries about why the world is fucked up. Honorable mentions are global warning, taiji dolphin slaughter and overpopulation.
Grab a webcam and create a huge long series on youtube. Make money and get your computer paid for.
If you don't do all of these at the same time, there's really no point.