On my final rotation currently, previous student walks in to drop off a gift to our preceptor and leaves... teacher goes "he must really want a good grade" after the student walked out.
I think a simple thank you card (for the preceptors that were great and challenged you to learn) is more than enough, and more meaningful. Hell, I only have done that for one rotation though (d/t job offer and good rotation). These brown nosers nowadays...