I have a set of people (friends and family) who I always send postcards to (at least once a year from some trip). rarely, i will send cards to colleagues or other staff. Most of the time, people love getting these cards.

As someone mentioned above, it could be difficult to find a postoffice in a foreign country.

Got one tip from a recent trip: In Spain, the Tobacco shops sell stamps. And they are in every street corner! Open late too!