When i am at work i use my grippers.I have rather a hands-on job anyway,and its allways get up and go.
If you sitting in the office for long periods of time surely you can go buy a strong gripper and weaker one hold the stronger one on the negative and do the weaker one for reps afterwards just do 4 sets.hold the strong one for about 12secs then immediatly do 6 reps with the weaker one.
I use heavy grip grippers they arent as good as the captains of crush but theyre grippers(i intend to buy the coc set as i get stronger)i use the 250lb for the negative hold and 200lb for my reps.
Also add some pinch grip work,find an object around 4iches in diameter and weighs around 10lb and grip and hold between the thumb and fingers (protein tubs full with protein i find ideal obviously holding by the lid)hold that for around 12secs too.
I train my grip three times a week like this on my training days afew hours before training,my grip strength has shot up using these techniques.
Hope that helps.
(I know my spelling is shit and so's the grammer but i cant be arsed to correct it as its 12.00am and i still have all of tomorrows food to make)