A bit late but here you are some suggestions:

You can add the St. George church, considered to be the oldest building in Sofia - it is located in the courtyard of the Sheraton Sofia Hotel. 

Battenberg Mausoleum (http://en.wikipedia.org/wiki/Battenberg_Mausoleum) and the little garden around it is a perfect place to eat a sandwich and drink a fresh from FreshBrake that is nearby (Shisman 12A street - I think you haven't been there because you haven't blog about it :) ) - one of the best places for fresh food in town.

Last thing, you can visit Arena di Serdica hotel (very close to the back of the National Art Gallery, http://www.arenadiserdica.com/en/contact-us/). It is build on top of some excavated ruins from the amphitheater of the Roman city of Serdica and you can drink a coffee there watching the ruins.

For even more inspiration, you can check http://en.wikipedia.org/wiki/Tourist_attractions_in_Sofia

Enjoy :).