ask and make suggestions:

• what do you want to say?
• why don't we ...?
• what about ...?
• can we choose this one?

give opinions:

• i think ... is a good idea because ...
• i don't think so because ...
• all right, i also prefer this one.

agree and disagree:

• all right, let's do that!
• absolutely!
• yes, i agree with you.
• i don't agree with you because ...