Most states' 529 plans are managed by investment companies such as TIAA-CREF, Vanguard and Fidelity. The number and types of investment options vary by state, and once you select your option you can't change it. You can, however, roll your money over into another state's plan if you're not happy with your chosen investment option. There is no penalty to roll the money over into another state's plan, and you can do it once every 12 months.
Many plans are also offering investment choices that are age-based. This means that if you're starting early, perhaps when your child is age one to three, the investments can begin aggressively in stocks then gradually shift to bonds and money market accounts as your child gets closer to college age. Some state plans offer several levels of options for aggressive, moderate and conservative investments.
If you can't reach the risk level you want in one plan, you can always open a second 529 account in the same or another state. You can have as many accounts as you want and can also contribute to both a 529 plan and an ESA. That way, you can diversify your investments in the event that the plan doesn't offer the investment mix you would like.