For the basement stairs: cabinets, stairs with flip up steps and very narrow stairs. Each step goes up one at a time for each foot. It is sort of spaced so you are putting one foot per step with a steeper step. Very space-saving.
Under stairs storage: Awesome and attractive - don't have the room or the funds for glass fronted drinks cabinet or display case? Build it into the understairs space, and it would be fairly easy to add lights.