<p>DH is a professor and over the years has received all manner of thank yous. By far the most appreciated are the thoughtful notes (and emails, but not quite as much) from the students he has advised and recommended. The sort of tchotchke mentioned by BlueLake are thoughtful, but tastes often differ and after teaching for decades (in husband’s case) you might guess that he doesn’t need one more. A more practical gift, say a Starbuck’s card, feels very awkward knowing what my husband makes and how little money his students often have. One actual gift that for some reason feels ok is when a student returns home, then brings back some traditional food from his country or state (e.g. Indian sweets or pralines from the south).</p>
<p>edit-- Just noticed that there is a second page of responses. Good idea with the chocolate bars. If I were lucky, I might even get a little piece. 
edit 2-- I think H would feel very uncomfortable receiving a $40 or $50 gift from a student, whether he was currently teaching him/her or not.</p>