It's more-or-less universally agreed that if Greece still had its own currency, devaluation could help the country recover from its current economic depression. It's also agreed that, lamentably, this is not possible without Grexit.
But there may be a way. Consider: devaluation works by raising the prices of imports (in the local currency) while reducing the prices of exports (in foreign currency). That is, a devaluation acts like a tariff -- a positive tax on imported goods and services and a negative tax (i.e. a subsidy) on exports.
Do you know what else acts like tariffs? Tariffs.
So if we want to emulate a Greek currency devaluation, we could simply impose a tariff on Greek imports and dedicate the revenue thus collected to export subsides. (And yes, it would probably be a good idea to have an outside entity involved in collecting and disbursing the money involved, rather than relying on local authorities.)
Now, like any option that might actually help the Greek economy (and, by the way, increase the probability that Greece's creditors will someday be repaid), this strategy will undoubtedly be ruled UNTHINKABLE by the Wise Old Heads of Europe. (Aside 1, Aside 2).
Of course, the real fun to be had by tabling a proposal like this would be to watch German politicians complain that a tariff along these lines (especially if it might be generalized to other depressed debtors in the Euro zone) would hurt German exports. (For an explanation of why this is so funny -- and it is -- see Aside 3.)
(Aside 1) The EU appears to have bred its own special variety of Very Serious People for which we need a name. I'm proposing "Wise Old Heads of Europe," or WOHEs for short.
(Aside 2) The correct response to anyone who tells you that something is "unthinkable" is, of course, "Think harder."
(Aside 3) For any particular country, net lending equals net exports. Borrowers run trade deficits, lenders run trade surpluses. (This is neither theory nor observation; it is an accounting identity, true by definition.) Therefore, for Germany to "collect on its debts," its trade surplus must suffer. So the ostensible goal of German policy towards Greece an other Euro debtors has always been to hurt German exports. The difference is that the direct application of tariffs might work.