If you are stuck at your desk and know you need to get up and mobilize your hip flexors, but just can’t get away, this is the next best thing.
Shift over to the right in your chair so that both sit bones are still on the chair, but your right leg can hang down. Now just let the leg hang, upper leg perpendicular to the floor, lower leg and foot behind you (on your toes). Sit up straight and hold a few minutes.
Switch sides and repeat. If you have a chair with armrests this is obviously not going to work, but as long as your chair has no sides, you can stretch out tight hip flexors while still being productive!