I like the ideas that you already have but you dont make the card formal since it would seem weird unless he's an English teach cuz then you can impress him with SAT vocab. Also its important it keep it short and concise without writing an entire essay.
But you can something like "Dear Mr. Soso, I would like to thank you so much for writing my letter of rec. You have always been there for me as a coach and teacher, and I am so lucky that i am able to get the chance to know you and have you be one of my favorite teachers. I will miss you so very much when the school year ends. But until then, thanks again and i cannot wait to continue to spend time with you this year. Sincerely, pinkunicorn17"