Glad you brought up this question. I was debating this as well. We decided if we ever bake cookies or such, we'll make some extra for my CFI and his wife.
On Tuesday night before Thanksgiving, the hubby and I baked some holiday cookies. On Wednesday, I gave a dozen to my CFI to share with his wife. My next flight was Friday, two days later. He said that his wife bakes a lot and he said "she appreciates quality baked goods". He said they both loved the cookies. I'm glad they enjoyed them. I also bake sourdough bread a lot and gave them a loaf two weeks back - they loved it too. It was a fairly large loaf, so they froze half. They ended up eating the entire thing in 4 days.
I like that these gifts have been well received and maybe as much value as a gift certificate. I feel it doesn't cross the line of being too much, but still lets my CFI know I appreciate his quality training time. Every since the loaf of bread, he seems much more attentive/patient to me and my questions.
I think they are looking forward to our Christmas baking sessions.
You haven't had sourdough bread until you've had it made at home with long bulk fermentation times (30+ hours).