Complete plans to build a built-in bed and seating bench. Look under the Home Improvement section for Bedroom Makeover.
With the right plans, materials, and equipment, you can construct a Built-in Wall Bookshelf as shown here.
If you would like to add a window seat to your home, try building one from ready-made wall cabinets. Be sure to select cabinets sturdy enough to bear the weight of someone sitting on them.