Smasrob Waterproof Bike Pannier, 30L Rear Bike Rack Bag with Multiple Pockets and Adjustable Clips, Bicycle Pannier Fits Most Bike Racks, Black (1 Bag)

I use panniers on both sides. I used to use this $46 double pannier which worked fine, but I disliked how much of a pain it was to mount, unmount, and carry off bike - I'd probably still be using it if I wanted to keep it on a bike full-time. I now have a pair of these $40 roll top waterproof panniers which are much nicer for taking off the bike.

I pack a laptop, clothes (except shoes), lunch, snacks, and all my water for the day (I like my filtered water from home better than the tap water at work so I pack about 3 quarts of water with me). Pannier on each side of the rack (clothes & water on one side, laptop & food on the other) plus a trunk bag on the top for my bike emergency kit (larger than it really needs to be). I've used a few different panniers. The one I like best is similar to Ortlieb Backroller.