Catch air while skating/biking/rollerblading down Laurel Street from Bankers Hill to the airport. Wave off the ambulance to make the whole experience free.
Drink a beer on the beach. It was relaxing when your parents did it, but now it's the quickest route to a thrilling encounter with the cops.
Storm the field... at the Balboa Park driving range in South Park. Balls screaming past at more than 100 MPH can curb withdrawals for even the most strung-out adrenaline junkies.
Go sidewalk-chalking in North Park. Nothing gets the blood pumping like risking a year in jail for playing with children's toys. (Google "Chalk-U-Py")
Cross the border into Mexico.
Sing karaoke at The Lamplighter in Mission Hills. The amount of adrenaline released is inversely proportional to the alcohol consumed. Try it sober for the best/most terrifying experience.
Sunbathe naked at Blacks Beach. Things get exciting when the regulars saunter over to chat.
Pull the trucks off of that old skateboard and drop in... to the stairs at your condo. Neighbors love this one.
Play Frogger at the Interstate 5/805 merge. Counting the bypass route, it provides 21 lanes of car-dodging exhilaration.
(Disclaimer: Probably don't do any of these things.)