Here are some expressions you can use to agree and disagree.

Agreeing

I (totally/ completely) agree with you.
Right!.
Yes, OK.
Yes, that's true.
I think so, too.
So do I/ me too.
Neither do I/ Me neither. 
That's a good idea!
That's sound great!
You are right!.
Absolutely!.
I see exactly what you mean.

Disagreeing

I don't (really) agree.
I disagree with you.
That's not true...
I don't think so, because...
You may be right, but ...
I don't know, actually, I think...
I know but I ...
I see your point but ...
I think I'd rather...
I'd prefer to ... 
I think it would be better to ...
I'm not sure about that.

Now, it's your turn:

You and your friend are planning a night out together. What do you decide to do?
( Don't forget to justify your opinions. Give reasons!)