Basic:
* The crowd erupted in spontaneous applause.
* The flowers spontaneously bloomed in the spring.
Descriptive:
* Laughter erupted spontaneously from the group of friends.
* She spontaneously burst into tears when she saw him.
With a reason:
* We spontaneously decided to take a road trip after seeing the beautiful weather.
* The music started playing spontaneously when the lights dimmed.
Figurative:
* His mind was filled with spontaneous thoughts.
* The idea came to me spontaneously, as if out of nowhere.
As an adverb:
* The party was spontaneously organized.
* The children spontaneously started playing together.
You can also use "spontaneously" as an adjective, but it's less common:
* The spontaneous reaction of the crowd surprised everyone.
The key to using "spontaneously" correctly is to understand that it means "happening without planning or premeditation."