Refusing an offer isn't always easy, especially if the offer is for a fun night out at an izakaya. However, if you can't make it, here are some techniques I use to bow out gracefully. Usually, I go for the easy let-down. I feel a too-direct, "Thanks, but I'm not interested," may be too straight-forward in all cases. Certainly, you can say it like that, but a softer approach is not a bad way to go, either.
> Cite a conflicting appointment. "Oh, that's too bad. I was invited to a farewell party for a co-worker at that time."
> Cite a family issue. "I promised my significant other I'd go out with him/her."
> If it is for a night on the town, you can cite a medical issue. "My doctor told me to take it easy for a few days."
> If it is a request for something that is truly bizarre or out of your comfort zone, a direct, "No thanks" is, of course, appropriate.