This really depends on your personal situation, how risk tolerant you are, what your other retirement assets look like, and what you expect you'll have for medical bills in the next 10 years.
I'm 36 and healthy, and I don't expect to need any of my HSA funds in the next 10 years, so I'm putting it all in the s&p 500 fund. It's the least volatile and lowest expense ratio stock option I have available.
Others may want higher growth or to keep it in cash if they might need it in the next few years.