A: Let's have pizza! B: (Say yes)
Good idea. / Great idea. / (That) Sounds good (to me). / (That's) Fine with me.
A: Why don't we go to the movies? B:(Say no politely)
I don't really want to. / I don't really feel like it. (You can add extra information here.)
A: How about having sushi tonight? B: (Say yes)
Good idea. / Great idea. / (That) Sounds good (to me). / (That's) Fine with me.