Here are few suggestions. I usually tell my clients to stand up for the most part of the day, walk to other co-workers to discuss business issues instead of email or calling them, take the stairs if possible instead of the elevator and if possible join a gym close to their office (unless their office provides one for them) so they can do a workout either before work, during lunch time or after work. Also, they should be watching out for those bad food choices that are available at the workplace.
I may have missed it, but I didn’t see anyone suggest a standing or treadmill desk. I use one for all the things I used to do sitting at a desk. I sometimes just stand at the desk, but most of the time I have the treadmill running at a slow pace. Pace really depends on the person and their familiarity with the desk set up. I do set these up for clients and teach them and my instructor prep course students in the use of the treadmill desk.