"To spontaneously do something" is to do something without having planned it beforehand. For instance: "One day during summer break, me and my friends spontaneously decided to go on a road-trip". "While i was taking a walk I spontaneously decided to visit my friend's house".

To happen all at once, with no preparation. We spontaneously decided to go to Paris. I sponataneously decided he was not the man for me.