Accepting and refusing politely may depend on what you are asked. For example

Would you like some cake?

Yes, please. Sure. Thanks Okay. Thank you

No, thank you I'd better not. No, but thanks for offering.

Would you like to go see a movie?

Okay. Sounds good. Sure. I'd love to. Yeah. Good idea

No, I'd rather not. I'm sorry, but I can't. No, but thanks for inviting me.

How about some more pie?

All right. Thanks Looks good. Thanks Don't mind if I do.

No, thanks. I'm really full. Thanks anyway. Looks delicious, but I'll have to pass.

How about going skiing this weekend?

Great. What time? Sounds like fun. All right. When and where?

Sorry. I'm busy this weekend. I don't think I can. How about some other time?