I mean, the problem here is that what you do on vacation should be what you want to do, so no one can really answer this question for you. You can use your vacation days to do literally whatever you want - why would you squander that doing something the internet thinks you should do? And more importantly, why don't you know what you want to do? Most people I know have every vacation day in their year mapped out way in advance, because they have so many things they can't wait to do with their free time.
And the reality is, a vacation doesn't have to be "fun" or "relaxing". Again, you can do literally anything you want. So if you can't think of what you want to do for vacation... maybe spend that time talking to a therapist to figure out why you don't seem excited about the prospect of actually being able to do whatever you want for a while.