All new furniture, paint, and artwork.
We started with lighter walls and custom furniture, including sit-to-stand desks for all employees. There are significant physical health benefits with adding sit-to-stand desks into offices.
(See article: https://www.huffingtonpost.com/thomas-b-trafecanty/the-benefits-and-consider_b_9996782.html)
Also, incorporating art into the workspace has proven emotional benefits including stress reduction.
(See article: https://mic.com/articles/106504/science-shows-that-art-is-having-fantastic-effects-on-our-brains-and-bodies#.YemXGgoN5)
There are so many factors that contribute to well-being and efficiency in the work environment. Design affects so much!