With a few exceptions, you can invest in just about anything: mutual funds, individual stocks and bonds, annuities and even certain real estate. The IRS does have a few restrictions for IRA funds, including lending money to yourself, using it as collateral for a loan and buying real estate for your personal use.
Similar to any retirement investment, it’s a good idea to diversify your assets among stocks, bonds and cash. Stocks can provide long-term growth potential, while bonds and cash offer some protection against market setbacks. For more on asset allocation, see "What's the best way to divide my retirement investments?"