Ice. Shiny smooth ice...?
What's more interesting about the shiny (i.e. reflective: I guess nobody thinks they're actually luminous) spots is not that there are so many, but that there are so few. Ceres must be collecting dust all the time, and that dust must be shoved around by micro-impacts. One would expect an ice surface to be covered rather quickly, so what is it that causes these uncovered ice regions in those specific places?