The free site Postcrossing lets you send and recieve postcards from random people all over the world.
Sign up to the site and send out 5 postcards to other members around the world. Once those members register they’ve recieved your postcards you are eligible to start getting postcards from 5 other random ‘postcarders’ across the globe.
Postcrossing itself is free, although you’ll need to pick up stamps and postcards on your own. The more you send (with a max of 5 out at a time) the more postcards you’ll receive. You can write about yourself, city, or anything else you like on each postcard and maintain a collection at home, on Flickr, or create a customized travel map for your blog or website.