Rocks. You can find them almost anywhere (Including Amazon! Seriously! You have to check it out.), and they come in a lot of shapes. The preferred shape for these is round and smooth, but you could try a rough rectangular one. One thing these all have in common is a single step: wash the rock before you use it. This is very important, because you don't who did what on those rocks. Bird poo can carry disease.
Use number one: Pet rocks. No, not putting the rock in a dollhouse and covering it with a blanket. This is actually painting and drawing a pet on a rock. Paint a base layer and let it dry. Next, use a pencil to draw the shape of a pet (Maybe even your real one). I drew a cat. Color your shape in with paint or markers that have good coloring and dry on paint, like Sharpies. Let dry.
waterproof sealer, your lovely designs will wash away when they get wet.