how much you like him. How much the money helped and how you'll always remember that he helped you when you were in trouble, and if you can ever do anything for him he should ask. And that you'll use your newly fixed car to drive to visit him, maybe using some of that extra money to take him to a nice dinner or something. Parents and grandparents need to hear from their kids and grandkids that the old folks are still loved. (I know you love him, but you probably think it goes without saying. Saying is always appropriate.) Just speak from your heart and you'll do fine in the letter ... but personal follow up would be golden too. Good luck. I'm glad you have such a loving grandpa.
