two twin beds. A curtain hung from that cute heading would disguise your "bedroom" if you chose. A curtain would work above as well for either a bed or built in shelves and hanging space for your clothing; I really love the idea of deep, built-in beds
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.