About mesodinium...
Postcrossing makes me still everyday curious if I might have a new postcard in my mailbox... :-)
In general I would say I don't have any preference but would be very grateful if you choose a real PICTURE postcard showing something typical for your country, region or city you're coming from. And if you're then telling me what can be seen on the postcard - great...! :-)
Typical comics or cartoons from your country would also be funny and great to receive!!
Other favorites are historical city views, historical city maps, nature views e.g. any sea side, old trees...
I hope that this might help you a bit for your selection.
Thank you and happy Postcrossing!