Well, besides the OBVIOUSLY ENDLESS killing sprees, I recently found a rather cool yet unofficial mini-game:
At Mount Chiliad (on the side facing the Radio Station in San Fierro), the slope is actually possible to run up (whats with that anyway?). If this is done with a high wanted rating (3 or higher) then when you get about half way up, the police cars start to drive off the top of the mountain straight at you. The trick is to avoid these large ranchers etc. as they plummet towards the water below. I once lasted thirteen of these attacks before an enforcer got me. For the best fun, do it with 6 stars and avoid those tanks!
Also, there is a really obvious spyware advert at this link. See it and laugh at their pathetic attempts!http://isg10.casalemedia.com/V2/38996/39654/