It's best to lie, and then find a new subject of discussion quickly. So you've got your easy standbys that most people know are a lie:
That thing? Sorry I wasn't there I was...
1. Stuck in traffic.
2. Stuck at work.
3. Being used for psychological experiments beyond my control. It was a crazy one. I'd tell you about it but it's classified.
At least those are mine. But you wanna deflect quickly:
1b. I fell in a sewer! Can you believe it?! A freaking sewer was missing its lid! ... Yeah, no, I'm ok now.
2b. I was installing Christmas lights and got electrocuted. ... No, I've never seen "National Lampoon's Vacation". What's that?
3b. I had to rescue orphans. From a fire. ... No, I'm no hero. They're the heroes, so brave. ... Well, I adopted some of them.
4b. I'm a doctor. Sometimes we get called. ... You didn't know that? Well, I like to be modest. ... Being a doctor? Well, I won't lie - some days it's hell. But when you save even one life, it makes it all worth it. ... Sure, I've saved many. Many many. ... How do I find the time to do that and my other job that you know I have? There's always time for saving lives.
And if you wanna add an extra bit of spite to your excuse, tell something that is obviously false but no one can call you on.
1c. There's a crazy blizzard outside. ... No there's not? Well, there is by my house, which is over 10 minutes away, walking. You wouldn't know, because you were here on time. ... It's summer you say? Well, maybe here, but at my house... Gee, global warming, huh?
2c. I turned into a chicken and then back again. No one will ever be able to explain how or why. Eggs anyone? (do come with eggs)
3c. Yeah, yes I was late... Anybody want cornbread? Just baked it. (do bring cornbread)
Also you can always say you had diarrhea. Just practice your dishonesty and you too can awaken the sleeping liar within. I believe in self-improvement.