For something that happens unexpectedly:
* Biglaan - This is the most common and literal translation, meaning "sudden" or "unexpected."
* Kusang-loob - This literally means "voluntary" but can also be used to express something happening without planning or prompting.
For someone who is outgoing and improvisational:
* Madaldal - This means "talkative" and can also imply someone who is quick-witted and able to think on their feet.
* Masigla - This means "lively" or "energetic" and can describe someone who is full of life and spontaneous.
* Walang pakundangang - This means "unhesitating" and can describe someone who acts without thinking too much about the consequences.
For a feeling or action that comes from the heart:
* Kusang-loob - This can also be used here, emphasizing the authenticity and genuine nature of the feeling or action.
* Taos-puso - This means "heartfelt" and can express a spontaneous act that comes from a genuine place.
For a natural phenomenon:
* Kusang-buhat - This means "natural" or "automatic" and can describe something that happens without human intervention.
Ultimately, the best translation of "spontaneous" in Tagalog depends on the specific context. Consider what nuance you want to convey and choose the word that best fits the situation.