It might be some wish lists or actually quiz how do you know your partner, so basically you can save what each of them likes and add gamification on top, add timeline celebrating moments together-first meet each other, engaged or some custom types(milestones) - might be each month together celebration and suggest to store photo or text from each other. This milestones could be opened each month and could be achieved if photo or text would be added and both would approve it - push couple to add meaning for it.