Spare dress shirt, underwear, small toiletry kit? Do you have a business-ready bag which can fit a few magazines in a stack? Yes? Just get a packing cube and put it in the bag.

I like the Eagle Creek Specter Tech with zips around on 3 sides and has thin handles on the ends. The slick fabric makes it easy to pull out of a bag especially if you tetris it with other similar material pouches. Some like Eagle Creek's packing folders which keep things less wrinkled. Lewis and Clark Electrolight is similar packing cube with a mesh front and "compression"/"expandable" zipper which helps if you're really not packing much inside.