I wanted to send her the message "Happy Belated Thirtieth Birthday!", which is 30 characters, via postcards, one character per postcard.
I found 30 post offices in unique places throughout the U.S. (For example, I found a town that had the same name as her given name in Illinois. I found another with my name, etc.)
I used Zazzle to print a custom post card for each location with a picture of the location on one side, and a large block letter on the other. I just did a Google image search at the time to find photos of the location. Here's the image I used for "Truth or Consequences, NM":
https://upload.wikimedia.org/wikipedia/commons/6/68/Truth_or...
I then hand wrote a message on each postcard that started with the block letter. This was because she'd be receiving the postcards one-by-one and I didn't want her to realize right away that a message was being spelled out. It would seem like a postcard I might send her from that location if I'd actually been there.
I also found a set of 50-state stamps the USPS had previously issued on eBay, and used the correct state's stamp for its postcard.
Finally, I round-tripped each postcard through its respective post office by mailing it inside an envelope addressed to the postmaster at that post office along with a note to the post master:
Dear Postmaster:
I am mailing my sister 30 postcards from 30 towns for her 30th birthday. I have enclosed a postcard, which I ask be hand-cancelled with a postmark from your town. To protect the postcard from machine cancels in its journey through the mail system, I have enclosed a stamped envelope addressed to my sister in which to seal and mail the postcard. Thank you very much for your time!
I wasn't sure if this would work, but damn if she didn't get all 30 postcards each properly postmarked.
I dropped them all in the mail in NC. Some went as far as Alaska and Hawaii.
She received them in Miami. I think she got the first one within a few days and the rest dribbled in over the next two weeks.
Edit: here they are after she received them all: