Mortgage buy down v. Investment?

Hi, My wife and I are about 5yrs into a 20yr mortgage @ 3% owing about $320K. I would like to put extra towards something but what? I understand I could invest and potentially make more but there is also the psychological aspect of not having burden of a mortgage. Another factor is I have about 12yrs until I could retire. On the investment side, are there types that are less risky that would allow me to invest but beat the 3%? Sorry for the easy questions, other than my 401k which I fund fully but don’t actively manage, I am new to investing. Thanks in advance!