Nice sentence! A little fix will make it sound best.
If you mean many monkeys:
Correct: Bananas and other fruits are monkeys' favourite treats.
Why? Because "monkeys'" (apostrophe after the s) shows more than one monkey. We use "treats" (with an s) because bananas and other fruits are many treats.
If you mean one monkey:
Correct: Bananas and other fruits are a monkey's favourite treat.
Why? Because "a monkey's" (apostrophe before the s) shows it belongs to one monkey, and "treat" stays singular.
One more small note: "favourite" is British spelling. In American English you would write "favorite." Either is correct depending on which kind of English you use.
Short version to copy:
- Many monkeys: Bananas and other fruits are monkeys' favourite treats.
- One monkey: Bananas and other fruits are a monkey's favourite treat.
Great job — you wrote a clear idea! Just pick which meaning you want (one monkey or many) and use the matching sentence.