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 caring man!

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 table? they mean something like:
What are we going to talk about?
What options, or choices, do we have?

This idiom or expression can have the following meanings:

Having a grudge:
Being unhappy or upset with someone or about something.
He has an axe to grind with his ex wife.

Also see:
Holding a grudge.

This idiom or expression means:

The first thing we need to do.
The first thing on the agenda.
The first thing that needs to be done.