If I were you I would buy things that are not avaliable in Poland or are expensive here.
When I was in the US, I used to love big Marshmallows, good Peanut butter, all kinds of Reese's (we don't have it in Poland at all), jellys, Newman chocolate.
For females you can buy some clothes - In Poland we don't have Abercrombie but people love this mark - you could buy normal t-shirts, but ask earlier about the size.
Every woman would be crazy about Victoria's Secret underwear - it's really hard to get in Europe. But I don't know if you will be interested in buying underwear ;)
I know that in the US there are much cheaper cosmetics of Lancome or Estee Lauder. You could buy f.e. Lancome Juicy Tube - lip gloss - it costs in poland about 120zł (about 40$) and in the US less than 20$. Or you can check brand MAC - their eye shadows (but not only) are known as very luxurious, they are really well-known here, hard to get and expensive. In US they are cheap.
Those gifts would be appropriate rather for young people, but I'm 20 and write about things I would like to buy if I went to the USA.