Cash on hand

Everbank - euro CDs (but their rates aren't that good)
you can also try the ETF - "FXE" - which gives both higher flexibility and better rates

cash under mattress is, imho, way too drastic

1. There is no reason to keep more than $100K in a single bank account unless you've got so many buckets of $100K that it would be difficult to manage the number of accounts (and if that's the case this discussion should probably be moot). Note that you can create multiple accounts at the same institution (you, spouse, joint, etc).

2. No. If we get to the point where you would need $20K in a safe at home, we've probably got much bigger problems than where the cash is and you should be more concerned with where your canned foods and shotguns are.

No, don't keep that much money at home. That is very risky, plus you are not earning any interest on that money. If you do keep the money at home, convert the cash to bonds or travelers checks so that nobody can use it if it is stolen or lost.

I would take all of the 100K plus and convert it into singles and then stuff my mattress with it and then sleep on it.
Seriously, I don't think we're looking at a debilitating '29 sorta a crash, perhaps just a slow insidious bleeding.
But as secondandcwhy suggests, why not create multiple accounts, I have two online accounts and I easily toggle between.

You could try gold coins under mattress, but then, they are very uncomfortable to sleep on

You can put all your money into singles and then use them as toilet paper when the dollar is worth less than the Peso! YEAH!