All cash.   Stocks and commodities.  

When things get volatile they nearly always restrict margins and shorting etc., which is saying that margins and shorting are indirectly part of the problem when a market goes from orderly to volatile.

No, margins and shorting are part of the solution.  Shorting actually helps stabilize markets, because as stocks are collapsing declining (inb4sherricksaystherewillbenocollapseandanyonewhothinksotherwiseisadoomerchickenlittle), people cover their shorts, thereby providing much-needed support.  It's just that our lords and masters want to blame somebody, anybody, for the rampant fraud and mismanagement of the economy, and blaming "the shorts" for falling markets today is a lot like blaming "the darkies" for rapes in 1870s Alabama –– get the lynch mob, and who cares that it was really the victim's brother.

Nothing will be as successful at convincing shareholders they should sell their stock like someone telling them it is illegal to short it.  It begs the question: why would people be motivated to short it in the first place?

Wasn't excessive and unstable margin trading a huge part of what brought on the '29 crash?

Do we learn from NONE of our mistakes?

Why would anybody cover their short position if the stock prices were crashing...?