re-purpose a crib mattress. Love this idea. I am SO doing this when the kids outgrown their crib/toddler mattresses. It wont be for quite some time, but by then we'll actually have a house so it can become a reading nook or something =)
Awesome Loft Beds--Since you won't have a window on the one end of your upstairs, this could be a fun idea for the kids area. Not necessarily a bed, but a 'loft' on top of a 'hide-out' area---or something. The bookshelf stairs are a neat idea too.
DIY twin bed with storage - interesting idea to make shelves or cubbies instead of drawers like a captain's bed - easier! On a Queen or King bed, would be a big gap in the middle, though - (HIDDEN STORAGE)