A bag for most seasons that won't break the bank.
If you are a daily motorcyclist without a car then the question you have to answer most (after folks look as you sideways when they find out) is how you buy groceries and carry stuff around. For a long time, I kept it simple with some sort of backpack but the Aerostich Courier Bag is my new go-to. It works equally well for carrying my iPad, keyboard, and camera or a heaping basket's worth of groceries. And when I'm walking around with it off the bike, I feel less like a high-school refugee.
The Courier bag is rugged and minimalist. For $97, you get a sturdy and simple bag made of urethane-coated 1000 Denier Cordura and lined with the yellow waterproof material dry bags are usually made of. It's not advertised as waterproof and the seams aren't taped, but you can expect your stuff to stay dry in a light drizzle. There are no dividers or compartments inside, and instead just a large versatile open space and two flat pockets. Aerostich offers an organizer pocket if you'd prefer things to be a little less minimalist.
Load the bag with your stuff, put it on and adjust the shoulder strap with the simple camlock adjuster. Buy the $9 stabilizer strap too, it works. Aerostich makes these bags in four different sizes, Letter (9"x11"x3.75"), Dispatch (12"x14"x3.75), Courier (12"x21"x7") and Parcel (14"x22"x9"). With a light load, the Courier bag wraps around your back and you can barely tell it's there. When it's crammed full of stuff, let the strap out so the bag sits on the seat behind you. Even if you over-fill it to the point that it wont velcro shut (I fit $80 worth of Trader Joe's the other day), the two buckles keep it closed.
Hauling around a 17" laptop sucks, so I use an iPad with a bluetooth keyboard instead. The Aerostich iPad sleeve is simple yet effective armor to keep your iPad safe while it's bouncing around with hard pointy things in a large open bag. It weighs nearly nothing, has no moving parts, and at $28, it's one the cheapest protective sleeves around. Made by Aerostich in Duluth, MN out of a proprietary three-layer laminate with nylon on the outside, impact material in the middle and brushed fleece on the inside. If you prefer a laptop over an iPad, these sleeves are available in a multitude of sizes.