You are complaining because its monday...give me a break I always get something at the last minute. Pearl necklaces and knee pads are great ideas, but I doubt she would think you bought them for her and not yourself. We have no idea what she likes, and therefore can only recommend universal things that girls like...chocolate, flowers, jewelry... I personally find that none of these things do any good, what you need to do is take the damn girl somewhere. YOu are obviously in high school guessing from the question, and girls die to take trips somewhere. Go to the beach, go to somewhere with nice scenery, just take her somewhere and buy her dinner, ice cream whatever. Girls are suckers for trips and evenings of conversations... I know it sucks, but grin and bear it.
You'll thank me, and I am sure she will thank you for the necklace later.