What a lovely idea !
-- leave a "blank day" page as a surprise
-- create puzzles - search word, crossword, jumble words, etc. of your self/relationship, etc.with clues re the same
-- prints of past emails & text messages exchanged
-- dried flowers, etc. from potpourris too - "you have scented my life" or something like that.
-- quotations on love, parting, etc.
-- recommendations on a movie, book, website, etc.
-- "commands" - eg. "go to the window, look out & write what you see/feel", "take a stroll down a certain path", etc.
-- jokes, riddles, posers, quips, ...
-- song lyrics, CD, etc.
-- a piece of your dress that he likes very much ; a lock of your hair ; your fav pen ; bookmark ; etc.
-- a "date" - choose a particular day & time in the future [maybe place too - some hangout you both like] & it would be like a 'remote' virtual date.
Remember, don't add Too much on each page: it can get overwhelming ;-) Keep the letters short, too.
Hope this helps?
Have fun :-)
Add : some more ideas :
-- A 'treasure map' - you can hide something somewhere & give clues & directions or a map so that he goes on the hunt to find that hidden object !
-- draw a maze quiz with "all" roads leading to your photo, which he has to solve.
-- a promise-coupon for something you will do for/with him when you return
-- a gift coupon to any store h e frequents
-- a ticket to some game/concert he would like to go to during that period.