I planted a dogwood and scattered my dog's ashes there to honor him. It was in the yard where he spent a lot of time.
I like the idea of plants because they feed the earth, as your pet's remains or ashes do.

Get several colors of bright acrylic paint and invite some kids to paint the markers for them. I did this with my grandkids for our 3 dogs as each passed on.

They smiled, cried, laughed and remember each one seeing the stones where they are buried. Little life learning lesson for them also.

My favorite one is the Lady Bug stone my youngest painted.

They have a beautiful memorial garden and you can get some ideas from that site. I like one idea of wind chimes.

You can mark and seal rocks, too.

If they make one smooth enough.

How To Transfer Photos & Digital Papers Onto Ceramic Tiles & Stone Tiles:

My dad chose cement for a marker for his cat. Not concrete (no additional sand or gravel), just cement (which already contains some very fine sand) and water.

Wooden posts with carved images and text are available commercially. They can even be laser-carved. Kind of expensive, unless you can find a local artisan to do a custom marker for you.

It's clear and used on wood and probably other materials like stone.