As open enrollment arrives, millions of Americans face key decisions, such as picking health insurance. If the plan comes with access to health savings account , or HSA, contributions, you could use the funds for long-term investments, experts say.

While most workers spend HSA money on yearly out-of-pocket health expenses, a small percentage invests the balance , which can grow tax-free for future medical costs.

"The plan is to go into retirement with a six-figure HSA," said certified financial planner Dan Galli, owner of Daniel J. Galli & Associates in Norwell, Massachusetts. When coupled with other Roth and after-tax retirement funds, "this is the holy grail of retirement planning," he said.

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