Go to Montana de Oro state park. Lots of good trails there. Off Los Osos road which is about 5-10 minutes north of Pismo Beach. I think some trails listed are on www.geoladders.com, so check there. There's also a lake near Arroyo Grande that has a park and has some good trails too. Good luck! Jeff
Lake Lopez is a quick twenty minute drive, from Pismo, through Grover City (now Grover Beach). It has some good singletrack trails and if you loop them together, some brutal climbs and mildly techy descents. Scott Tinley puts on a great weekend (festival) of MTB and regular triathlons there every September-ish. Info on the lake: http://www.slocountyparks.com/activities/lopez.htm Info about Tinley's Dirty Adventures....I think they have since removed the word "dirty" http://www.tricalifornia.com/index.cfm/Tinley2011-main.htm
The park has a daily use fee just to let you know. I think it is $5.00 but you can go to the waterpark after your ride and cool off.
if you're driving 20 minutes anyway, Montana del Oro is more worth it than lopez. the trails are just better. there's also a bunch of stuff in San Luis Obispo (10 minutes north on 101). do some searches on this site, there are tons of threads about rides up there.
The two trails I hit when I went to Pismo almost 3 years ago were: Montana De Oro - http://www.geoladders.com/show_route.php?route=261 - As started above, this one is way cool and my favorite. Some tough climbs but at one point your close by the beach so the views are great. Would do this one again in a heartbeat. Madonna Mountain - http://www.geoladders.com/show_route.php?route=273 - Directions weren't the best but I found my way. Decent trail though. There was a nice rock garden in the mix. Good enough were I'd go back.
Also as stated, Madonna Mountain is a great ride, it is litterally right off the freeway. Great views when you get to the top. Montana de Oro is only a 20 minute drive west, from there and riding and views are the best around. Me personally go do Madonna and MDO it is far better than Lake Lopez. Lake lopez would be on the bottom of my list.