If you have any old plant pots or other old pottery that you want to break up (and make sure there were never any chemicals used on them) - you can make a cave or other hiding spots with broken plant-pots.
In general - if it's something that:
1. Won't Rust
2. Doesn't contain any harmful chemicals
3. Won't leech any chemicals into the water
4. Isn't so sharp that it could hurt the fish
It can be used in an aquarium.
"To know that we know what we know, and that we do not know what we do not know, that is true knowledge." -Confucius