I'd like a tree like this and heart shapes cut out of paper. Guests can write their well-wishes for the couple on the hearts and attach to the tree with small clothespins. Later, the hearts could be put into an album or grouped together in a frame.
Family tree wall: Self-stick tree decals are available in many designs online. Scan old photos (It's quick 'n easy with iPhone or iPad + Pic Scanner app); print and frame. Download the app free at the App Store.