For more ideas, go here: http://www.ldrmagazine.com/blog/2015/01/22/put-inside-open-letters/
A pretty good outline or guide to explain what goes into a "Open When" letter
Some of these are a bit far fetched, but I think these are super adorable and original and would definitely bring out an interesting side to each of you
When this traveler proved her heart was still with her boyfriend when she slipped this into his pocket at the airport : | The 23 Most Thoughtfully Romantic Gestures Of 2013
Kisses in a Jar | 20 Things That Will Help You Through Your Long Distance Relationship -- LOVE this!!!!