There is no question in my mind that the USD is never going to become worth much more, and it is always going to lose value (long term). This is obvious to anyone who understands the scam called fiat money.
However, virtually all the currencies currently in use are fiat money just like the USD, and it is highly likely that they are going to lose value faster than the USD at some point. My personal opinion is that the "bottom" for the USD relative to the EUR, JPY, even the CAD, AUD is near. Think about it. Americans have recently fallen victim to the "American dream of home ownership" scam. BFD, look at the problems the other nations are getting themselves into. They may not be as overleveraged in the mortgage market, but ultimately the relative value of a fiat currency is determined by more than just a real estate bubble. There is socialized medicine, hostile tax environments, laws restricting international trade, and plenty of other aspects in which some (in my opinion, most) countries are far inferior to these fine United States.
I lived in Europe for years, and I have lived here - in the USA - for years. The reason why I decided to live in the United States of America is my firm belief that, when the shit hits the fan, the people in this country will come out ahead. Americans have a strong sense of independence and are highly motivated and productive if they are presented with a worthwhile opportunity. They are generally more willing and better able to adapt to changing circumstances; they don't sit around expecting the government to tell them what to do; they take the initiative and try to navigate around whatever the government may throw their way.
Now that you have listened to all this general babble, let me answer your question directly: gold and silver.
Edit: If you must trade a "conventional" FX pair, you might want to check out the GBP. All the world's central banks are subservient to the Bank of England. This may or may not mean that the GBP is going to collapse last. Frankly, I don't care to look into this any deeper than I have. I already know all I (personally) need to know about the GBP. It does not meet my standards for capital preservation, but yours might differ.