I am fairly new to this, but I have seen some set ups with flat sided pottery laying on the bottom to hide in, like a terra cotta pot. Are you growing plants in there? I have also seen a tube cut lengthwise to make a cave with cocoanut hair and aquatic plants tied to it with thread until it all roots together. I thought that was neat looking. I have a moss ball in my tank and Michigan kicks it around. I am not sure if he is playing with it or not when he does that. I think giving them some sand to dig in will be good as well. My little buddy was trying to dig and burrow into rocks before I realized the sand would be better. I wouldn't do the rocks at all. They sometimes swallow the small ones, and the big ones let too much stuff fall though to the bottom and that turns into a nasty mess. I also have an old, but safe, mug nestled in my tank that he swims into sometimes. I try to keep soft edges to avoid damage to his feet.

I hope something in here is helpful!