Hi Richard,
As an ex-Disney cast member and opening team trainer of California Adventure, I can tell you about this. When Disney opens a new theme park the opening team received a medallion like this to celebrate the occasion. I have medallions from every park I helped open including Euro Disneyland, Animal Kingdom and California Adventure. I also have gotten the ones for Tokyo Disney Seas, Disney MGM studios and Hong Kong Disneyland from friends in Imagineering. Some are worth more than others. The California Adventure one is probably lowest in value going from $20 to $30 each. Euro Disneyland I’ve seen go for about $75.