I really like my Tubus minimal QL3 rack and Ortlieb Downtown pannier/briefcase. It is not the lightest means of transport, but the rack really is “minimal” and barely visible on the bike. They also sell the rack and a “city bag” if you don’t need the Downtown
I like a Deuter Race Air EXP rucksack when I want to take a nice/race bike, but the Tubus/Ortlieb is lovely for my normal commute on the Paddy Wagon.
EDIT: It is worth saying that the QL3 system leaves all the protrusions from the back of the pannier on the rack, not the bag. This is perfect for a work bag. They have yet to make formal QL3 panniers, however, so the minimal rack really is just for the briefcase/city bag. I have a set of backrollers too for another rack, and a QL3 bracket mounted to the trike, with rack bag on top.
Topeak make a laptop bag that will fit onto their detachable seatpost MTX rack, but I haven’t tried it. Downtown on the trike with MTX rack bag for usual lock and gubbins: