If you plan on getting rid of the wood supports you can do a bookshelf with a built in ladder on the far side to get to the books and make use of stuff there and aren’t afraid of falling and on the near side put a couch with some sort of support so it can’t fall into the stairwell, but only do those two things if you’re ok with some clutter and difference in how that stairwell is set up, otherwise put something there to stop people falling regardless, thus the couch with the supports.