Perhaps an interesting railing-iron or tempered glass panels set in iron to maintain that open feel. I would not want to obstruct the view to the tall glass panels on the right.

A railing (for sure) and an interesting open back wooden reading chair.

Add either a small chest or wine cabinet in that area. Have fun with the area.

Not gonna get away with no railing, as that's a code issue if you live anywhere civilized. I'm really surprised that wasn't an immediate requirement as a condition of sale from the previous owner. Your home insurance will likely require it as any visitor could easily break their necks trying to go down those stairs.

For the nook, a sculptural chair that is art object and utility object in one would work well.

A bookcase.

Two benches with pillows for a little conversation area.

Perhaps a couple Panton Visiona stools?

http://hivemodern.com/pages/product7121/visiona-stool-verner-panton-vtira

...or a Donald Judd chair would look amazing here.....

http://www.google.com/url?sa=i&source=imgres&cd=&cad=rja&uact=8&ved=0CAoQjhwwADgp&url=http%3A%2F%2Fmoussemagazine.it%2Farticolo.mm%3Fid%3D49&ei=rgxqVeujD43VoAS09IKIBw&psig=AFQjCNF-Ky0IpsO6yawSOmgcps3QJMorTg&ust=1433099822340080

If you built the house, then the railing should have been an issue in order to get a certificate of occupancy. Then you could take the railing out, but it would need to go back in if you decide to sell.