This is a killer abs workout you can do while sitting down anywhere. If you are at the office and have an office chair with wheels you might want to be careful so you don’t fall over.
Position yourself on the edge of the chair and grab hold of the seat just under your legs. Do the crunches by lifting yourself from the seat and bring the knees to your chest. Repeat! Beware because this is quite an heavy exercise but I know you can do it.