I once heard the concept of treating yourself as if you were a child on days when you need to rest and recuperate. I think it’s based on the idea that everyone still has an inner child that needs to be looked after in some way, but sometimes we’re so overwhelmed by having to be an adult (cleaning, cooking, studying, emails…) that we neglect that side of ourselves a bit.
Depending on how you're feeling on your self-care day, you can literally just do exactly what you want - your dream day - but in a way that’s measured and will allow you to slip back into your routine for your upcoming exams.
For example, my dream day during an exam season would be waking up relatively early, having a really nice breakfast, reading a book in the morning and then having lunch. After a quick call with someone who I love, I would then go swimming, before going home and watching a favourite movie of mine while eating a few of my favourite snacks. Then I’d just prepare a few things for slipping back into my exam routine and go to bed pretty early.
I guess the point is to try and see which part of you needs to relax more (your body, your mind) and to allow that to happen. I hope that this makes sense and that it’s helpful to you! And good luck with your exams :-)