at desk

ab flexion
butt flexion

kegels

mini tricep dips using your arm rests work

basically, focus on contracting said muscle group, holding, and then releasing. build up to longer holds.

jog in place, but only if you are wearing a ninja suite. Ninjas go unnoticed

Do belly rolls when you are not typing! Move your belly in and out and until you are either out of breath or you can feel your stomach getting a work out. You will be surprised by how much of a work out this really is. Your stomach muscles will start to get sore but it will burn a little calories while your sitting down reading emails, articles or browsing the internet.

Exercise One - Side Stretches
To help whittle your waist and keep your spine aligned and supple, side stretches are excellent exercises to do at your desk. With your arms raised over your head or akimbo with your hands on your hips, lean gently over to each side as far as you can safely go. Please use caution if your chair is unstable or if it is on wheels. You can do this a few times on each side at several times throughout the day.
Exercise Two - Arm Raises
In order to help protect against carpal tunnel syndrome, poor circulation, and weight gain, you should do arm raises several times a day. There are different methods of doing these simple exercises: raising your arms straight over your head, raising them to the sides, or even behind you. Each method works a different set of muscles, and all will temporarily increase your heart rate, keeping your metabolism revving.
Exercise Three - Butt Squeezes
The easiest exercise that you can do at your desk is butt squeezes. You can even do them while you are on the phone or typing up a report. Simply squeeze and release your buttocks muscles several times. This is the number one exercise that will help prevent "office chair spread" of your hindquarters.
Exercise Four - Leg Lifts
Of course, these are not the type of leg lifts that have you lying down on the floor and swinging your legs around. At your desk, you can easily straighten your knees and lift your legs out in front of you. You can also march with your feet in place. This will exercise the large muscles in your legs. For easy calf exercises, you can raise your feet up on the toes and lower them. Be sure to use your muscles when you do these exercises for maximum effect.
Exercise Five - Stress Relievers
Many of the exercises already described here will really help to relieve stress of your desk job. However, there are some other great stress relief exercises you can do at your desk.
Gently stretching your neck, or rolling your head around, will reduce neck and upper back stress. Lifting your arms in front of you and bending your hands up and down at the wrist will help to relieve the stress of typing and help prevent carpal tunnel syndrome.
As you spend year after year hunched over your desk for eight hours every day, you may find your muscles atrophying, and your hips and waist spreading. Not only can you help prevent that weight gain and loss of tone, but you can alleviate stress with these five quick exercises. Exercising at your desk only takes a few minutes, and will do your body and mind a world of good.

Deskercise! 33 Ways to Exercise at Work
http://greatist.com/fitness/deskercise-exercise-work/