There are many custom grille badges that can be found. Club membership, home country, etc. They are basically classy bumper stickers.
Mercedes offers their own high-mileage awards. They are free, all you have to do is print out a form, fill it out, get your odometer verified at a authorized MB dealership and mail it in. In 2-4 weeks you'll get your badge(s) via FedEx.
250,000 kilometers (155,000 miles)
500,000 kilometers (312,000 miles)
750,000 kilometers (466,000 miles)
1,000,000 kilometers (625,000 miles)
You can download the award application here