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":
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:
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:
What she did was got an atlas and identified every town whose name was at all romantic. (Things like Loving, New Mexico.) The postcards that she had come back appeared to be from random girls with as different as she could make the handwriting to be. It was all timed to show up on Valentine's day.
After getting two stacks of postcards from girls with cryptic messages like, "I hope to see you again" and "Visit any time", my brother got a reputation for being a total stud.
And how long did it take her to realize what the message was?
It's been eight years. I think she texted me after she got the first few. I don't recall when she realized there was a message. I seem to recall they came in in batches. She's get none for a day or two and then get three or four. I also recall there wasn't a lot of correlation between how far they travelled and when she got them. It probably mostly depended how long it took the post master at each post office to re-mail.