1) The old, "oh, sorry, I've got to go to the toilet" excuse. Don't forget your bag, because it'd be really awkward if you had to return to fetch it.
2) "I've got to move my car, I didn't pay for enough parking!" This one is a good one because you can literally go to your car and drive off. Byeeee Felicia!
3) Text a friend, tell them to call you, then answer the phone in shock and say you need to go, your friends having an emergency, she just broke up with her boyfriend and you've just gotta go! Or whatever, another excuse you can think of.
4) My mum's just called, she's locked herself out of the house and I'm the only one who has the key, it's a unique key! Say this whilst getting out of your seat, creating mid-action movements startles the other person, they won't have time to have a come back because by the end of your sentence you've already walked out the door shouting "IT'S A UNIQUE KEY... I ONNNLLLLY HAV...IT" and cue fade out Homer Simpson style. Or here's another with this dilemma, get your acting skills on, quick smart. Think a totally dramatic thought, gasp and say " oh my god, i've locked my poor cat outside" politely again, cue fade out.
And finally, you could pull a diesel move, just get up and RUN.