Oooh. This is a good question! I had quite a few friends pay off student loan debt in the last couple of years and the question always comes up, "What next?"
First off, kudos! You're about to accomplish something many Americans struggle with. You're now in a great place to start looking at the things you always wanted and/or know you needed to do. For example, do you have an emergency fund established? It's always great to have at least 6-12 months of living expenses saved. Most Americans don't have even have $500 in emergency savings, so it's wise to look at this as an area to build should anything happen!
Second, are there any short-term or long-term savings goals you have? Maybe you want to put monies toward travel, gifts, family, etc. Third, perhaps investing? That's a great way to grow your money overtime. Fourth, if you have a cause that's near and dear to your heart, giving back is a great to use your money and can also be budgeted for.
When I finished my student loan payoff, I decided to continue to maintain my monthly budget. I do my best to plan for everything, including savings, investments, travel, etc. This helps keep me from that feeling of "I don't know what to do next!" because every dollar has a goal behind it even if it's just the goal of "fun money" you can spend on the things you enjoy.
You've developed good consistent habits during your debt payoff journey -- don't deviate from it now! Once you're no longer paying off debt, keep persistent and learn to grow your money through investments, side income, etc. Perhaps, more importantly, learn to keep your money. Create and maintain those savings goals, especially the emergency fund. Having a little bit of reassurance is never a bad thing!
Curious to hear what others think!