http://www.hostels.com/en/hosteldeta...telNumber=1856

I haven't stayed there, but the reviews look good for a downtown hostel. It's right in the Gaslamp Quarter which the "IT" section of San Diego right now. Looks like they've even got private rooms for ~$28/night. I'd agree with the recommendation of a hostel. You can make a reservation online via hostels.com (the link above) and they do appear to have availability that night.