You can sterilize the gravel. Soak it in a bleach/water solution. Just rinse it very well. You can also boil it on the stove for 15 minutes, let cool, and rinse. To make caves you just need some aquarium safe silicon. You can use pvc
pipe, plastic pill bottles, or any plastic container that is aquarium safe. Coat item with
silicone and roll in the gravel and let it cure.
Old gravel makes good drainage gravel for flower pots. You can put it around flowers to act as mulch. Make a draft guard for a door by sewing a tube out of fabric and fill with gravel. So many uses.