As with most collectibles, low supply can equal increased demand, and therefore value. With that in mind, check to see if any of the seemingly innocent looking base cards might secretly be rarer parallels. You probably won't be able to identify cards which are simply short-printed unless you're very familiar with the set they're from, but serial-numbered cards have their print runs stamped right on them.
Print runs of 25 or less can be considered a loose cut-off for turning otherwise uninteresting cards into items with some value. Always hold on to any 1-of-1 cards you come across. They aren't the automatic jackpot they once were thanks to their widespread proliferation, but they will draw some interest when put up for sale more often than not.