(What you are going to do is…) or (What you are going to do are…)

It's an unusual construction that used in certain contexts.

For example, if a child tells a parent:

I don't want to go to the shops.

The parent might respond:

After I tell you a secret, you are going to want to go to the shops.

That's to say, you are going to change your mind, to see why visiting the shops is an attractive option.

In the song title, the writer is saying that whatever happens there is so attractive that all your desires will be focused on returning there. It is the only thing that you are going to want to do.

That's to say: going to want to do is the same as will want to do.

If you want to advise, then say "You'd better ...". If you want to instruct then you can say "You must ..." (or "You'll have to ...").

So both your examples are correct, but mean different things. I would only use "must" if I was in a position of authority. For example a teacher might tell her students that they "must do their homework", or when describing an external obligation "We must pick up the litter, it's the law!"

You would normally "tidy up" and not "clean" the picnic area, since a picnic is outdoors, and you can't clean the soil, but you can tidy up.