If you are looking for long term returns and plan to keep your money in place for 20 years plus, long term growth funds are great. Especially now since the market is a low. Check out the Fidelity and Janus family of funds.
Medium range return 7-10 years, I would seriously consider the US Automakers. GM is down to $4 bucks a share, I think Ford is around $3. If you have faith in their survival, they can only go up.
Also, you can't go wrong buying a house, especially with the market so low. There are some really good deals out there. In fact, if you wait another 2 years, the market will be bottoming out. Unfortunately for many homeowners, there are a lot of adjustable loans that will be resetting at higher payments in the next 2 years. The result will be many more foreclosures, which will result in home prices dropping even lower.
Oil is too volatile, unless you really know what you are doing. Between the sagging economy and alternative fuels, the prospects aren't too good anyway.
Gold is traditionally stable, but is at record highs. Remember the old saying, buy low sell high. Not so sure the timing is the best for gold right now. Diamonds, same thing.