The Berkeley Hills have two awesome concrete slides tucked away among the winding roads and large trees. BYO cardboard (or wax paper!) for riding. Have you tried them?
Codornices Park | Slide is curvy fantastic and pictured above. Super fun. Skinned elbows likely. Near the Rose Garden (walk through the under-road tunnel to get there). Gross bathroom. Lots of grass and room for walking.
Dorothy Bolte Park | 40-foot concrete slide is straighter, steeper, and shorter than Codornices. Easy parking if you know where to look.