If you live in Europe or if most of your investments are in Europe and illiquid, you are screwed anyway and there is no need to read further. If you correctly predicted the crisis a year ago and invested accordingly (and I will be first to admit I am not one of them), you are rich already and there is no need to read further. This targets individual investors that have relatively diverse and global portfolios that want to take advantage of, and more importantly, limit downside risks in the growing Euro crisis.
The obvious strategy is to short the Euro or buy derivatives or ETFs that have the same effect. In the long run, the Euro should be at most at parity with the dollar even without any country exiting. In the short run, its movement is small and unpredictable. The currency market is different from other markets in that it is extremely large and include not just investors but governments and corporations that need to trade currencies no matter how much they stink from an investment perspective. The currency market is also subject to government intervention and these days governments don't hesitate to intervene given strong national interests, and in some cases, political interests. This is exactly why UBS lost $2 billion after the Swiss central bank pegged the Swiss Franc to the Euro to prevent it from appreciating and hurting exports. It is also possible to use derivatives or ETFs to make levered short bets on the Euro but they are either not available, or are too expensive on a relative basis to individual investors unless you are right most of the time. As such, my recommendation here is to reduce Euro exposure primarily by buying Dollar and Dollar-denominated assets.
Forget about European stocks - even your grandma knows there is a Euro crisis which is already priced into European stocks and in particular, European bank stocks. Like last year, US and Asian stock markets will continue to have no direction which makes it extremely difficult to profit. If you have ideas or information on specific stocks, definitely go for them but be cognizant of the transaction costs and liquidity
I have strong reasons both for and against gold. On one hand, I don't trust governments and central banks and gold is something that they can't control by printing more or giving speeches. When governments collapse and money is worthless, gold is the only place to be. On the other hand, gold just stares at you but doesn't generate income. In general, gold is a good investment when 1) interest rates are low, liquidity is plenty and other investment options are bad, such as during Fed quantitative easing in 2009-2011 and 2) real catastrophe happens and the US treasuries no longer serve effectively as safe havens. Unless you think there will be a global financial crisis more severe than the 2008 one or World War III, I would not go all-in on gold.
Commodities other than gold
Europe produces very little of the world's commodities. Any major effects on commodities will be the result of government and central bank interventions due to the Euro crisis.
Cash and cash-equivalents (i.e. US treasuries)
Right now I think cash in dollars and dollarized currencies (e.g. HKD) is king. While savings accounts and treasuries offer paltry yields, inflation is still low and the treasuries market is unlikely to weaken and will even be bolstered by a worsened Euro crisis.
All in all, play safe and hold lots of cash. Invest strategically and opportunistically. This is not a time to make a big killing but to conserve cash and wait for the next big chance.