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.
IKEA Hackers: Wall of EXPEDIT: Workspace Plus Storage Plus TV. Love this idea for playroom upgrade - room for 2 kids, plus room for small TV and chair. Lots of storage. Desk can be used as wrapping station too.