On some days when I'm carrying more I will use a Brompton O Bag clipped to the front carrier block... Holding a change of office clothes, extra idevice charger, iPad, helmet light, water bottle
If I'm only carrying my scrubs which are relatively light, aux battery charger, and iPad then I use my Brooks Barbican messenger bag which feels weightless when using the waist strap and makes transitions from folding bike to train very quick.