I’ve run into a few situations where the work sort of dries up and I have more free time than normal. But in most places I have worked, I end up with a UX backlog that keeps me busy during the driest of periods. Those times when I don’t have a backlog, I almost always fall back on a few different strategies.
There is always research to do and if the budget allows, I go out into the field. If the budget is slim, I pare down the research or might conduct a competitive review depending on the market I am in. You can work on professional development too — learning a new tool, trick or method. And I almost always have a side project or two that aren’t on a strict deadline where I can put them on the backburner if things get busy. But if you are at a small company, I kind of doubt you will end up with a whole ton of free time.