Thank you for one of the most enjoyable visits we have had in many months.
Thank you for one of the most memorable days of my trip.
Thank you for doing so much to make my trip to New York interesting (pleasant)(resultful).
Thank you for contributing so much to the pleasure of our stay in ...
Thank you so much for your generous hospitality.
I hope something will bring you to New York soon so that I can reciprocate your kindness.
You must give me the chance to return your kindness when you visit here.
Thank you very much (ever so much) (most sincerely) (indeed) (from the bottom of my heart).
Many thanks for your kind and warm letter.
Thanks a million (ever so much).
Please accept ( I wish to express )my sincere (grateful) (profound) appreciation for...
I sincerely (deeply) (warmly) appreciate ...
I am very sincerely (most) (truly) grateful to you for ...
There is nothing more important (satisfying) (gratifying) to me than to receive one of your letters.
Your letters are so much fun (comfort) (entertainment) (company).
Your most courteous (considerate)(delightful)letter...
I cannot tell you how much your letter delighted(relieved)(amused)(enchanted)me.
I love the way you say (put) things in your letters. You make even the smallest incident seem so interesting(important)(charming)(mysterious).
It was good (fine)(charming)(thoughtful)of you...
It was nice(characteristically thoughtful) (more than kind) of you...
At the outset, I want to thank you for your kindness to me and for your compliments.