What an interesting contrast in attitudes on this subject between CFIs and airline pilots.
In a recent thread, it was suggested that airline pilots should get tips, and the resounding answer was that "professionals do not work for tips". That accepting tips would degrade the airline pilot to the level of a waitress, limo driver, or other somesuch unskilled laborer.
So, I find it of note that so many CFIs -- who always are quick to remind people that they, too, are professional aviators -- in this thread seem to be pro tips.
Firstly, I don't think any CFIs expect
tips. I've gotten two as a CFI and was surprised both times. More accurately, CFIs occasionally receive, "'Thank You' gifts." "Work for tips," is another beast, entirely.
Secondly, flight instruction is a one-on-one, personal service, something airline travel typically is not. As a student I felt the need to recognize the efforts of one of my instructors who went above-and-beyond to make my experience a good one. As an instructor, it's likewise nice to know your extra efforts are noticed and appreciated.
Point being: we're not comparing apples-to-apples here.
Finally, my impression is that the OP wants to get something for his instructor, but wanted to make sure doing so was okay and, if so, get an idea of what would be appropriate. You have consider our responses within the context of the question. In that sense, the thread title is misleading.
If you want to parse it more accurately: Does one tip a CFI? No. Is it okay to get your CFI something after completing a course of training to show your appreciation? Yes, though it is by no means expected.