I recommend the Timbuk2 Tandem Panniers. Waterproof, built really well and very easy to carry of the bike. I have Ortlieb and Racktime panniers for touring but the Timbuk2's are my go to for commuting and short trips for shopping, Starbucks/Panera runs, etc.
They are saddlebags so they're attached and great off the bike because magnets keep the two parts together and an included shoulder strap makes them very totable.
Good luck.