Crazy straws for plants to grow around. Plastic canvas covered with aquarium silicone and rocks that can be shaped into any configuration you want. Craft lace jungle for fry and bottom fish to hide in. PVC
pipe glued in different configuration for ghost shrimp, loaches and other fish to hide and play in. Plastic beads for the fish to play with and hide from you. Plastic beads tied on fishing line for curtains or tank seperators that some fish will push through but others won't.
Will add more if I think of anything