Haha I've been posting a lot on this recently (i.e.
this thread) so I felt I had to chime in
The above recommendation to stand while you're on your computer is the easiest and arguably best suggestion. Place your computer on a book shelf, table, or other surface where it's at a comfortable height to type while standing up (in other words, your back is straight and you aren't hunched over). An even better version is to set up a workplace treadmill- there are lots of how-to guides online. The premise is you walk slowly (1-2 mph) while working, which over the course of the day burns tons of calories.
You could also sit on a stability ball while working, which many people find uncomfortable at first, but quickly adapt to. It's very good for the lower back and core.
If you don't feel comfortable with either of these options, you could also incorporate regular moving around and NEAT principles into your day. It is recommended to get up every 20 minutes to walk around. Do stretches, work on laundry, etc. Keep a pair of resistance bands and a YouTube video pulled up on your computer and do desk exercises. (
This one is perfect for sitting in a chair). Take the dog for several long walks throughout the day, walk while calling a friend or making work calls, etc. Fidget while you're at your desk. Anything to continue movement throughout the day is great to raise your activity level.
These are just a few ideas, other people made some suggestions on the thread I posted above. Good luck!