Maybe you could plant a tree or plant to mark the grave? I think it's a lovely idea. On my dog Lottie's grave we planted heathers because they're evergreen. It was nice, because it didn't look like a grave, so it didn't make me sad to look at, but it was nice to mark it in some way. And then when we had kittens around they played in the heathers and I knew Lottie would have loved them, so it was nice.
When we were kids we used to paint rocks with photos of our cats who died and write their names, but that did look very child like, and the paint wore off after a few years.