Most Helpful Girl
Keep it small. A parfum, or perhaps a nice soap. A bodywash, or some earrings if she likes small jewelry. Doesn't have to be expensive. Could also buy her a cute scarf, for when it gets colder in the winter, if it even does.
Most Helpful Guy
I was never a very gifty person, but maybe a cheap (but still nice looking) necklace. Something like yhat, or books, games, DVDs something she would enjoy but isn't too costy