I'd love to come but the in-laws are in from out-of-town.

I have the flu and it might be contagious.

I wish you had mentioned it sooner. Unfortunately, I'm busy that night.

We can't find a babysitter/dogsitter/grannysitter.

I'm on maneuvers then. If I tell you any more, security regulations require that I kill you.

I'd love to come, as long as you're cool with the fact that I'm a smoker. I really can't live without a couple of cigars an hour.

No can do. The moon is full that night, and I have to stay home for everyone's protection. Please don't ask me to explain.

I have to finish writing a major presentation for a business meeting scheduled for first thing tomorrow.This is what I get for leaving it until the last minute.

We just bought a puppy and he howls whenever we leave him alone.

I never go out when Mercury is a retrograde phase.