Yeah rocks are very docile and barely ever hide. They will normally stay where you place them, and they can come in so many shapes, sizes, and colors

For a 20L, here are some suggestions:

-Hognose snake
-Children's python
-Some type of small constrictor?
-Frog
-Salamander
-Leopard gecko
-Baby Bearded Dragon (40g minimum when adults)
-Corn snake
-King snake
-Garter snake
-Some type of colubrid?

I don't know what kind of reptiles or amphibians you like, so you gotta tell me. Just like how I love reptiles but don't really like tortoises or turtles much.