Drop-in Childcare recommendations in Oakland?

Hi. My son 3.5yo goes to a lovely prek, but they have random Fridays off at least once per month for parent-teacher conferences, staff training, etc. I do have a few babysitters I can call, but that ends up being pretty expensive. Does anyone know of an affordable drop-in daycare in Oakland or Alameda? He's fully potty trained. 

Thanks for any leads!