To go bananas

This expression has several different meanings. 

When someone goes bananas about/over something, he/she becomes very enthusiastic about it. 
The expression is considered slang and is generally used in informal contexts. 

Here are a few examples:

• The students went bananas when they heard that Mani was going to address them.
• We all went bananas when we saw Steffi Graf walk into the gym.
• The children went bananas when they saw all the icecream.

To go bananas can also mean to drive someone crazy or mad. 
When used in this sense, "to go bananas," means the same thing as "to go nuts" and "to go crazy". 

For example,

• The constant questions made him go bananas.
• Ana thought she would go bananas sitting all alone at home.
• Being in the company of her son was driving Mala bananas.

"Cool bananas" is an expression, which is frequently used in American English as an exclamation. It is often used to mean "wow". Here are a few examples.

• Satish got an A in the exam. Cool bananas!
• You've got a new bike? Cool bananas!
• Cool bananas man! That is really wonderful news.