I do pushups at my desk.
I can knock out a set of 15 pushups inclined at my desk in less than 30 seconds. 2 or 3 sets a day ... whenever I need to stand up and stretch from the computer.
Tricep dips off of my desk chair are good too.
Standing in place lunges to stretch my back and legs.
Squats (i.e. deep knee bends), while reaching up to the sky ... helps lengthen my whole back.
There are a lot of yoga things you can do at your desk too, that don't necessarily raise your heart rate or build muscle, but will help keep you loose and limber.