Ways we enrich our relationship long-distance vary. But one thing we've always done is read matching devotionals. (Yes- even before Art knew Jesus as Savior & Lord he read a devotion with me! Keep planting seeds, Ladies, if your husband is unbelieving still).
My Aunt gifted us our first set of matching devotions when Art left on his first deployment to Afghanistan in 2003. And we've adopted that idea as ours over the years. We have found this method works well. It helps to keep us the same page spiritually in some regard and it gives us something to connect with mutually.
The pages are dated so it's a great place to jot down important dates in his copy to remind him of events, birthdays, etc.
In my copy I also jot down important milestones for the deployment- the day he left. The half-way point. The homecoming, when applicable. It's a nice reminder of God's steadfast presence in this time.
It's also a great place to stash some notes with sweet nothings. :)
Our choice this 4th deployment cycle is Jesus Calling: Enjoying Peace in His Presence by Sarah Young.