hi- I am going to tease you and say a photo of your boss!
But if you want a recommendation for the walls of your office I would suggest some plant or plant that flowers that thrives indoors like african violets or some fern/orchid.
Also some peaceful little water fountain you can get at a DIY store like Home Depot..they are available under $100.00 and also along those lines would also be a small fish tank....the better choice would be a fresh water tank for easier less costly maintainence.
these bring life into a place and create a sense of peace and tranquility, beauty. peacefully moving and growing
Trees.......definitely. I like the self colour Yantra too.
Interesting that trees should come up. During my coaching training we did an exercise to find out which points of your life had the most successful outcomes when you felt most joy. We picked a series of ages with one event from each age and narrated any significant feelings, visuals, sounds etc. In each case, regardless of the situation I had the distinct feeling that trees were involved, even if it was something like passing an exam.