I'd sent a nice long card to your preceptor, letting her know in detail
just what you are telling us. Its something she will keep and re-read many times. Would also send a copy to your NM so she is also aware.
As far as a gift, for a 6 month preceptorship, I think $50. is appropriate, if you can afford it. A gift certificate for dinner, a massage, to a uniform
shop, subscription to a nursing journal she enjoys, gc for CD/books, manicure/pedicure. There are so many nice items you can buy, but the well written card is most important.