You know what?

How is it said in Russian?
for example,

You know what? I don't care!

You know what? Why don't we all just go?

You know what? I think you're just plain stupid!

1) Знаешь что? Мне всё равно!

Or you could use more colloquial expressions:

Знаешь что? Мне по-барабану!
Знаешь что? Мне пофиг!


2) Знаешь что? Почему бы нам всем не разойтись?

A colloaquial way of saying it:

Знаешь что? Почему бы нам всем не свалить?

3) Знаешь что? Мне кажется, ты полный дурак (полная дура)!

Thanks dnldnl.

Although I have to mention something, my second example is a little ambiguous mind you, it could have two meanings.

First is, why don't we go altogether,

the second is why don't we all leave. or why don't we just go there in which the main point is going there rather than going altogether.

If I'm right I believe you provided the translation for the first meaning.
Could you also provide a translation for the latter?

Jono, actually, the way I read your second example was: "Why don't we all just leave." That is, all these people are gathered in the same place and one of the people offers that they all leave. So that's the way I translated it in my first post.

Why don't we all just go (there)? - Почему бы нам всем просто (туда) не пойти? This translation is just as equivocal as the original English sentence because it could also mean to go altogether, depending on the context, but not necessarily.

Why don't we just go altogether? - Почему бы нам просто не пойти вместе?

Hope this helps.

Ah yes, that's right, I get it now.

Alright thanks a lot that surely helped.