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

Would you like some cake?

Yes, please.

No, thank you

Sure. Thanks

I’d better not.

Okay. Thank you

No, but thanks for offering.

Would you like to go see a movie?

Okay. Sounds good.

No, I’d rather not.

Sure. I’d love to.

I’m sorry, but I can’t.

Yeah. Good idea

No, but thanks for inviting me.

How about some more pie?

All right. Thanks

No, thanks.

Looks good. Thanks

I’m really full. Thanks anyway.

Don’t mind if I do.

Looks delicious, but I’ll have to pass.

How about going skiing this weekend?

Great. What time?

Sorry. I’m busy this weekend.

Sounds like fun.

I don’t think I can.

All right. When and where?

How about some other time?