This idiom or expression means:

On the agenda.
Available options.
Available for discussion.

“What’s on the table?” could mean any of the following:

What do you want?
What are you selling?
What are you offering?
What are your conditions?
What options, or choices, do we have?

A. Are we going on a trip or not?
B. I didn’t know that was on the table!!

This idiom or expression means:

Having something other than the obvious in mind.
Doing things for reasons other than what is announced.

A. I don’t know why Fred is suddenly interested in helping the poor!
B. He has a personal agenda. It’s not as if he is suddenly a different man!

Also see:
Have an axe to grind.

This idiom or expression means:

On the agenda.
Available options.
The things to talk about or make decisions about.

When someone says “That wasn’t on the menu,” they mean something like “I didn’t know that. I didn’t agree to that.”

When someone says “What’s on the menu,” they mean something like “What are we going to talk about?”

This idiom or expression can have the following meanings:

Have a grudge:
He has an axe to grind with his ex wife.

Have a personal or hidden reason:
If he’s offering to help you with your situation for no apparent reason, he must have an axe to grind.

Also see:
Have an agenda.

This idiom or expression means:

The first thing we need to do.
The first thing on the agenda.
The first thing we need to deal with.