How to say no in the really polite way

How to say no in the really polite way. Ok. it is very important.   for example.  First one.  Will you go out with me this Saturday? That sounds great, but I just can't put one more thing on my calendar for the weekend. I will think about it .so let me call you  on that day.  The second one.  Let's go a party / Let's go to eat dinner / Let's go for a walk  in the park /  Let's go to a movie. It sounds fun, you know, really interesting, but I will be busy tonight.  thank you very much, anyway.