Some good words to use in any thank-you note include "Thank you" and "Your kindness is very much appreciated." No matter the gift or favor, be gracious.
Keep the letter simple. Thank the person in the first sentence, then add a few more details. For example: "Dear Susan, Thank you for your generous gift. It is truly appreciated. I am grateful for your friendship and look forward to seeing you again." Or, "Dear John, Thanks so much for sharing in our wedding day. Your presence meant the world to us. It was wonderful to see you, and we hope to share many more happy memories with you." Close with "Thanks again" and regards such as "With love" or "Sincerely."