Those who plan to use this money to pay current medical bills are best served (as with all money you plan to spend in the short term) keeping it in a FDIC insured savings account. But, you can choose to invest your HSA anywhere. Such as in index funds like VTSAX. Once you reach the age of 65, you can withdraw your HSA for any purpose penalty free, although you will owe taxes on the withdrawal unless you use it for medical expenses.
