I like to use the smaller card format for collages. It lends well to organizing themes, and sure does make it easier if I spill paint on one.
This 14x18 shadowbox collage was completed in the summer of 2011 for dear friends who were getting married. There is an art card for how they met, where they were honeymooning, and their interests (including sci-fi and fantasy). Interspersed are elements of their faith, their wedding day, names, etc. The overall color scheme is gold and neutral along with the purple and burgundy of their wedding colors.
The frame opens up on hinges, so is easy to either see the art close-up, or add elements in the future. For example, a canceled stamp from their wedding invitation was added, and someday, a lock of their baby's hair will adorn the art. The shelf at the bottom of a shadowbox is a great place for tiny 3D objects to be set.
I consider this type of collage to be a beautiful way to showcase a person or a couple, but it can also be a dynamic piece of artwork, which can be altered and updated by the recipient to include future mementos.