What is the best way to securely invest a condominium or homeowners (HOA) ociation reserves by the board?

Our condo ociation definitely has the same challenges you face. Our reserves are fully funded, but the costs escalate as our community ages. We currently have laddered CDs. I do not believe that there is any other investment which would satisfy the fiduciary component of your board stewardship. Obviously a short-term CD ladder would be the way to go now, as the rates are so abominable. You can get insured muni bonds, but insurers have gone bust in the past, and I wouldn't want to risk that at all. Also, unless you hold the bonds to maturity you risk having them worth less than you paid for them. Hope this helps and good luck!