Me too! But mines twice as deep. We are thinking of doing shelves but making them look interesting. So some ideas are thick wood shelves that are hanging from wires, floating shelves maybe, or iron rods holding shelves. We love using wood that is rugged to give texture. But just can't decide yet. I plan on using mine for photos and art. No matter what it's an awkward space I'd like to see put to good use

I agree with SandyC fill it in with drywall. Putting items in the space makes it a focal point and most rooms don't need another focal point. Also a solid wall will give you more flexibility with furniture placement.