For my upcoming birthday trip to Vermont I'm going to make a mailed lettersheet journal. Before I go I'll make lettersheets (letters folded up and mailed without envelopes) because that way the postmarks and everything are preserved on the reverse of the letter. I am going through loads of states because that's the cheapest way to fly, and my two hour layovers will give me a chance to hunt down mailboxes and get postmarks from Michigan and Los Angeles. Then, because I am a novice traveler and am flying into the wrong airport, I am going through loads of states via bus or something once I get to the East Coast, and that will provide more postmarks. While I am in Vermont we are going letterboxing, so I'll stamp the letters with those as well, on the outside fronts if they are small enough.
I think it's okay that this is a journal to myself, a collaboration with the post offices of a half dozen states and VT's letterboxers.