Simple:
* The party was spontaneous and filled with laughter.
* She had a spontaneous urge to sing.
* Our decision to go hiking was completely spontaneous.
More complex:
* The band's spontaneous improvisation brought the audience to their feet.
* Her spontaneous outburst of anger surprised everyone in the room.
* The spontaneous combustion of the old building caused a massive fire.
Figurative:
* The spontaneous growth of wildflowers in the meadow was a beautiful sight.
* The conversation flowed spontaneously, making it feel like we'd known each other forever.
Remember, "spontaneous" can describe actions, events, feelings, and even things that grow or happen naturally.