More specific subject is children, and of course the vintage spirit is essential!
In my ATC cards the children sail to marine adventures and enjoy about summer days. :-)
Edit: Just went through some of the other beautiful cards posted for this challenge. Seems like ATC cards have inspired especially Finnish craft bloggers, and some of them made their first ATCs. Hopefully we'll see more from them... I have been making ATCs for about five years and have got somewhat fond of the concept. :-)
By the way: Just two weeks ago I found a new challenge that focuses on ATCs. It's Fun With ATCs.