海边那场即兴音乐会真是个奇妙的盛会，吸引了上百人自发参与。

The impromptu concert by the beach turned into a spontaneous ___happening, drawing hundreds of people to join in.

他们在公园里举办了一场即兴的偶发艺术活动，观众可以随意参与创作。

They organized a spontaneous ___happening in the park where the audience could freely join the creative process.