I don't remember very well, but I had several pretty amusing letters from both WW and NL.
In WW, I remember Rocco telling me about how "I found this under my sofa cushions and I thought you might like it". In that letter, he gave me a pear.
Another time, Filbert said something like "I found this in my bathroom. Here, but next time you have to dance for it! -I'm a hammer! -Filbert." I forgot what it was that he gave me, but I found the letter fairly amusing.
In NL, I've stored up plenty of interesting letters. I'm too lazy to find them, so I'm not putting them up here.
Well, it's not a strange letter. But, once.. When one of my villagers left, he sent a letter stating all the good memories we had.. I never spoke to him.