Ventana Wilderness if you are good at navigating through brush, fading trails and steep, rough terrain... a fire went through there and closed it for a year, and some of the trails are still in various states of disrepair - not to mention the latest round of heavy rainstorms has battered and degraded trails that were maintained recently.

Henry W Coe state park would be a good one - tons of trail, much of it steep but all pretty well marked. The best map available is from the park itself, coepark.org. There will be a ton of wildflowers and most water sources will be good. Since 20-30 miles is fairly low mileage for a 5 day outing, and there are good campsites fairly close together in this park, it might be worth a look.

The southern portion of Los Padres NF might be do-able, though there may be snow in parts of it - it will be a dusting of snow, not like the Sierras, if anything. There are also some nice hot springs down in the mountains near Ojai. This is what the other poster who mentioned Frazier Park and I-5 is referring to. There's a condor sanctuary out there. Google for Willetts Hot Springs or Sespe, there are some trip reports that will give you an idea of what the area's like.

Point Reyes would be good this time of year, so would Big Basin in the Santa Cruz mountains. I'm thinking of doing the Skyline to Sea trail myself, it's about 30 miles.