You say “just for fun,” but I think it is a great suggestion for real. Hope you don’t object to a serious answer.
I can certainly see a place for a declaration of friendship. It can be a wonderful thing to put your feelings of friendship into words and not reserve those expressions just for lovers and family.
I have told people, or written on birthday cards and other greetings, such things as these: that I treasure their friendship, that our long history as friends is very special to me, that I value them, or simply that I love them dearly. I haven’t followed any sort of formula but just spoken from the heart, every now and then, not too often, but just often enough to remind them of my sincere regard. And I have been deeply honored to receive such sentiments myself from time to time.