I am opposed to gifts with significant monetary value. They may be unethical for the recipient to accept, and even if not, can leave them feeling uncomfortable about the gift. Even $5 as a gift card. And come on, gift cards are like handing someone money but telling them where they have to spend it. Double awkward.
A sincere expression of thanks via card is good. I personally like to give handmade items. I made a funny framed needlepoint for a resident who was incredibly helpful to me. Materials cost less than $5. Probably about 2 hours of my time to make it. I like the symbolism of taking something of little intrinsic value and making it have meaning and aesthetic value through application of time and skill. That is essentially what happens in the training/education process, and so gifts that reflect that are appropriate. Ideally, a little craft that conveys a particular idea, maybe referencing an inside joke or some point that the recipient had really stressed.
Or, you know, a coffee mug is good, too, if you aren't crafty. Something that the person can keep and use for years, to remind them of your time together.
Ultimately, though, it really is the thought that counts. Demonstrating that you cared enough and were grateful enough to make the gesture. That is what will make the person feel good.