In my opinion, one of the best ways to show appreciation for your CFI (or any other person who teaches for a living) is to respect his or her efforts on your behalf by showing up for every lesson on time, with the homework done, and prepared to learn. CFIs have to put up with a lot of students who quit partway through, who don't want to mess with "the books", or who are just not serious about learning.
Also, in a small way, your doing your best and passing your checkride the first time is a "gift", since a good pass rate contributes to an easy renewal for the CFI.