Please, don't use envelope; make picture of the postcard for this site if I forgot to do it by myself; and don't be shy to write me a message about all important things of your postcard.
I like postcards with:
- the best tourist attractions
- the UNESCO Heritage Sites
- different alphabets, flags, maps
- all kinds of transport
- books, libraries, writers: especially classical
- healthy life style
Also I prefer stamps (marks) about postcrossing.