Gravel, coarse sand, perlite, marble collection basically anything inert that deoes not readily float. There are many people here who use ground tires. They sell them in Some Walmart stores and Sam's Club warehouse stores to use as mulch around plants and shrubs. They are about the same size as hydroton.
Big bucket of small lego's works well, does not hold any water so good for hydro constant drip.
Lego's? yep legos from walmart! always funny to see peoples faces when they notice.......
Alot of teachers use legos when teaching youngsters about different farming and gardening techniques.