The classic answer is municipal bonds, preferably through a diversified mutual fund or ETF. A municipal money market mutual fund would work best as a cash equivalent. Since your state does not have an income tax, you can choose a nationally-diversified fund rather than one focused on a specific state.

You'll want to run the numbers to see whether the (typically lower) returns on the municipal fund give a higher after-tax yield than paying federal taxes on other options. But I imagine a 35% federal rate is high enough that you'll benefit from a municipal bond fund.

Since you already use Fidelity: https://www.fidelity.com/mutual-funds/fidelity-funds/municipal-money-market