When we have an idea to do something together we can use one of the following forms:
Let's go to the beach!
Shall we go to the park?
What about watching a film?
How about ordering a pizza?
Why don't we go swimming?
When we think something is a good idea we can respond with one of the following expressions:
That's a good idea.
That sounds good.
When we don't think something is a good idea we can respond like this:
I don't really feel like it to be honest.
I'd rather not. I'm a bit tired actually.