A postcard makes room for what someone noticed while you were elsewhere.

The size of a postcard is the point. There is no space for an account of the trip, so what fits is one small thing somebody saw and thought of you during, which is a much more personal transmission than a full report would be. Constraints of that kind tend to produce affection rather than information.

Send one to somebody without waiting to go anywhere interesting. Write the single thing you noticed today that reminded you of them. The brevity will read as attention rather than haste, because that is what it is.