What is the meaning of "I would do for a candy bar"?

I am reading Diary of Wimpy Kid (Diary of a Wimpy Kid – Book 2 – Rodrick Rules). But when I read to this page. I get stuck about like that

What is the meaning of "What I would do for a candy bar" in this situation?

We had to do the whole skit in Spanish. Rowley asked me what I would do for a candy bar, and I said I’d stand on my head.

Here is my research(https://dictionary.cambridge.org/dictionary/english/candy-bar) candy bar: candy shaped like a thin brick. But It seems not to make sense for this sentence.

Would you like for explaining the meaning of this in the situation?

Thank you in advance!