sounds like you've maxed out your tax advantaged accounts, now the next on the investment order is opening up a taxable brokerage account and putting that $500/mo in there and investing in a vanguard index fund. It's easy peasy to set up and contribute, we have biweekly contributions automatically pulled from our checking account and automatically buy shares of our selected index fund. FYI, I remember you need a $3k balance to purchase most funds, so until you reach $3k, your money will be in a money market fund until that point, then you just sell and buy the index funds you want.
If you are the type of person that dislikes the feeling of debt hanging over you, I could also be swayed in the direction of paying your mortgage off faster, but the math strongly favors investing rather than paying off a 3.5% mortgage (unless you still have PMI).