For our third, we bought a picture frame that said "grandchildren" or something similar, and had different openings in it. We put one picture of each of the boys in a slot. Then for the last opening we took a picture of a hamburger bun sitting in our oven. It took my mom a minute to figure out what it was and get the whole "bun in the oven thing", but once she did it was a big hit.
This time around she made it easy on us. It was Christmas, and my folks were getting ready to head up to see us. She commented that if we had any more kids she was going to have to buy a SUV to carry all the presents. So dh went out and bought a Matchbox SUV and we wrapped it up and gave it to her Christmas morning. She got that one right away!