Tourism would probably do well because Greece would just become just like any other cheap Third World tourist destination for "First World" travelers.
Unless and until it stabilizes not too many people outside Greece would accept the Drachma. That doesn't mean trade would cease. Trading companies would set up barter deals. Greek vacations would be cheaper and, presumably, more people would go there to take advantage. A cheaper currency should also make the Greek merchant marine more competitive.
Actually The bank note printing companies are hiring. They are printing more money every day. It’s boom time.
I have a hundred trillion and fifty billion note on my desk. Quite attractive, quite worthless.
Have you met any people spending their vacations in Zimbabwe recently?
Greek tourism is, by far, their main industry. Greece is a country of unmeasurable beauty, visited by millions every year (as opposed to Zimbabwe). This will never stop. People (myself included) will never fail coming to Greece and leaving their hard earned dollars, rubles and the rest in exchange for Greek hospitality. Yes, it’s going to be hard for Greeks and cheaper for the “First World” travelers for a while, but at least they will be in charge of their own destiny, not their slave masters in Brussels.
Greeks aren’t Zimbabweans. Have you ever met a Greek person who didn’t know how to make a living?