I need a basic story idea?

NaNoWriMo (national novel writers month) is coming up, and altho ive wanted to do it for the past 5 years, this is the first time ive really had the time. ive been wracking my brain trying to come up with a basic plot, but nothing is coming. i want it to be light and funny sci fi, nothing serious. i dont expect this to be good, because i havent written in about 2 years, in fact ive never written anything unless it was for a grade, and i was given a basic direction, but i really REALLY want to. and i just want to use this as a chance to exercise my literary muscles. so please help.