Listen to your heart and you'll soon know what to write.

If it were me I'd write something like this:

"Too often we forget to acknowledge the friends in our lives and I just want to take this opportunity to thank you for .....blah blah"

Put some appropriate humour in it also like "I remember when....."
and put something funny in it. If I was dying I wouldn't want to receive cards that were all doom and gloom. But that's just me because laughter is such an important part of my life.