Wow, it really comes down to your goals and risk profile. I am a fan of copper and natural gas right now- but that is not really a good bet for a retirement plan and the intermediate term. I will confess a penchant for solid, real investments right now. China and India are buying them, and buying companies that produce them- China is trading USD for resources, and I would be tempted to follow suit. Personally, I would not touch anything housing related, commercial real estate related or anything in the financial sector right now.
One of the huge problems right now is the low returns of any traditional, safe investments. Bonds are paying very little (and the value drops when interest rates increase- something that almost HAS to happen, eventually.) I would go with a small percent in gold or silver (traditionally 10% is recommended to hedge against inflation.) However, where they sit right now I keep thinking they are a little overvalued- and the Fed seems to be predicting more deflation. However, if you are in it for the long run...
Being as you are in America, you would also have a "Treasury Inflation Protected Security" (TIPS) as an interesting option. You can find a good write up on what they are here: http://www.finpipe.com/tips.htm
The problem is back to my statement that traditional investment vehicles are paying nothing right now. As a matter of fact, 5 days ago the return on TIPS fell to a negative yeild (see http://en.mercopress.com/2010/10/26/us-treasury-bonds-fetch-negative-yield-ahead-of-fed-s-actions-to-prop-inflation
In English, it means that you are actually paying the government $5.50 for every $100 that you lend them. Yes, you are paying the government for the privilege of lending them your cash (and hoping that inflation breaks above 1.7%- the break even rate...)
At your age, I would think that a little risk in the hopes of higher returns would be a good option, but again- I don't know how risk adverse you are when it comes to finances. I would honestly say that if you have anything approaching a substantial amount to invest you need to talk with an independent financial advisor. When you talk with your bank they will steer you to mutual funds that benefit them the most (they get money to sell to you and do NOT have your best interest in mind.) When you talk with an insurance salesman they get residual "kickbacks" on your policy (every year that you leave your $$ with them they get a percentage to keep- they do NOT have your best interests in mind.) Even when you talk to somebody who is independent they will likely attempt to profit from your investments. I wish I were more positive- but people are scum and will try to profit from you whenever possible.
One thing to keep in mind are companies that pay dividends are often a very good investment over the long term. In addition to possible appreciation in the stock price you get a yearly payment for holding them (that you can re-invest.) They are often know as DRIP(s) [Dividend Re Investment Plan(s).] Many of them are pretty safe "blue chip stocks" like Coke. Watch for some of the utility companies who have old infrastructure or commercial real estate investments ect...
***The usual small print of this not being advice, not being a solicitation to buy or sell and steering you to consult your investment professional apply***