This is a perfect combo of my two favorite ideas for that space under the stairs leading to the basement. A reading nook AND book storage and shelves . assuming that there is enough space for this there.
The hammock is a great idea so smaller "secret playroom" areas. This kid’s bookshelf opens to a secret reading nook. A SECRET READING NOOK. I would’ve never left. (from Everything About Secret Bookcase Doors)
DIY Cardboard furniture That doesn't look like it's made from cardboard! We've had some great posts on cardboard furniture you can buy that doesn't LOOK like cardboard, but most DIY options resemble the brown recyclable stuff.