The last time I went to visit the grand-kiddos I estimated the number of days between when they'd get the letter I was sending and my arrival date. Then, I lightly glued little planes (cut from credit card promotional materials) to represent the days. (I made sure to have a few extra planes at the front end so that when the kiddos got the letter they could already remove, or X-out, a few of the planes.)
The children took turns taking off, or X-ing out, a plane. It was like a "count down to Christmas" only this time the countdown was a "count down to Mom-Mom" coming to visit.
The children love getting different and unusual letters each week in the mail.