Well, things I like. I like a little bit of everything.
I am part of a lot of fandoms, like Harry Potter, Star Wars, Doctor Who, etc.
I like anime and manga, especially Trigun, FMA, Death Note and Shaman King to name a few.
I read a lot, almost exclusively fantasy books. I love the Wheel of Time series, The Song of Ice and Fire, The Otherland series.
I love cats, and have one lovely cat at home.
My favorite animal is a badger, so anything with a badger on it is a definite win!
My favorite fantasy creature is probably a hippocampus, with mimics coming a close second.
I am an avid gamer, both video and tabletop. I have a delightful collection of colorful dice, and as an RPG player, they make me very happy. I love action platformers like Psychonauts and Jak and Daxter, as well as a good puzzle game like the Professor Layton series. Persona 5 and the Soulsborne games are also a treat indeed!
Regarding postcards, I don't really have a particular preference. I love all manner of beautiful or interesting things, be they landscapes, drawings, or colorful patterns. I will be happy with any sort of postcards.
I do love weird things like aliens and Lovecraft, horror art such as that of manga artist Junji Ito, or things with a Tim Burton-esque style. I would be terribly excited to see something like that in my mail box.
Food postcards are always so beautiful, especially when they are showing off local treats! I love tea and doughnuts, so those would also make me smile.
As for what to write, anything! Tell me about your day, or your pets, or some fun plans you have coming up! Just write something! I think getting to read the postcards is as fun as seeing what the picture is, so just fill it up with whatever!