SEPs are able to hold a wide variety of investment options. SEP IRA investment choices are determined by the financial institution at which your SEP IRA is established.
When opening a SEP IRA, be sure to find out what investment options each financial institution offers, and what the charges for various transactions are. Some custodians will only offer a small range of options, or it may be limited to their proprietary mutual fund family. Others will allow you to invest in any securities that they can trade on their brokerage platform.
You may want to enlist the help of a financial professional to help you get the plan set up and to advise your employees when they need it. Generally, a SEP IRA should give you access to a large universe of securities: stocks, bonds, ETFs, mutual funds, REITs, and so on.