These are adorable! ^__^
My suggestion would be a letter-writing themed stationary. With pencils, stamps and stamps (How do you define the difference between a send-my-letter-please-stamp and a put-this-on-an-ink-pad-and-stamp-stamp?), envelopes and such :)
I think like postage mark (like the round date sending one with the ~~~~~ lines next to it), would look really cool on stationery!