If you get a book which has just one topic, and lots of illustrations (like a book about birds, or mountains, or car restoration, or something!) then you can incorporate the illustrations, and it'll havea cohesive theme. (So you can have the birds singing words... and perched on a picture of your house, or migrating from your house to his, you get the drift...).
Alternatively, atlases work really well - you can mark on special places, places you'd like to go, places which are really hot (but not as hot as him), or plot out your relationship history as a journey across the world, rather than through time (don't forget to leave some places undiscovered)
That's an awesome idea...I'd love to do that for friends, too.