the 'best' is usually something like slate tiles as it's more natural.
Other options are: newspaper, paper towels, ceramic tiles,
the ones to avoid are usually: crushed walnut shells and calcisand as they CAN both be fatal if swallowed in large quantities.
Playsand is the fighing talk of substrate; not recommended for babies but 'ok' for adults although the risk of impactation is still there for sloppy eating beardies. You'll have the people who use it, the people who recommend it, the people who don't use it but don't think it's too bad and the people who hate it.
One thing for sure with playsand is that you'll come away from a playsand thread more confused than when you entered it.