Increasing physical health significantly increases mental health in my experience.
Biking + Water = joy!
Fitness: Ride a bike! See the world! As you are already indoors most of the day staring at your computer screen (as I am) I highly recommend taking up an "out-the-front-door" outdoor physical activity that you can do for the rest of your life.
Biking is low impact on your body and you can get an incredible amount of fitness in a very little amount of time while taking in fresh air and sunshine. I have burned over 1,000 calories in one hour and over 10,000 calories in a day while traveling over 150 miles on a bike. It doesn't get easier, you just get faster.
As a side note: fat leaves the body through CO2 on your outbreath! WOW mind blown!
Fuel: WATER!!! Drink water & eat real whole foods. You should always have clear urine so drink water first thing in the AM and all day long, drink enough to take a piss break every hour to get you up and out of your work chair. This alone will make you feel 10x better.
Healthy fueling happens in the grocery store. Shop the edges of the store (not the aisles) and buy things that contain lots of water and are a single ingredient: vegetables, fruits, bananas, apples, carrots, dates, watermelon, etc. not items that are processed. I have a sweet tooth and after eating fresh bananas and dates I don't even touch/want the ice cream in the freezer. I swear, dates are the most amazing food on earth! Take them on your bike rides instead of processed bars. Eat as much fresh raw food as you want, your body processes it quickly and you won't feel bogged down like when you are digesting meat.