Paw prints and name would be nice. You could also have a cartoon version of him done, not exactly a picture/portrait. (If you like the cartoon idea I can refer you to a person that will draw it for you.) What was his favorite toy? Is there a certain path you walked or tree you sat under together often? The possibilities are endless. I also like the idea of collar & tag.
Sorry for your loss. =( It will get easier with time, I promise.