Now that the weather is (finally) warming up, messenger bags and backpacks are going to start feeling a bit warm for commuting, and in some cases leave their sweaty outline behind on your favorite bike-to-work shirt.
The way around this, of course, is to make the bike do the carrying, by using bags and racks. But what if you don't want to leave stuff hanging off your bicycle while you're indoors?
The simplest solution is to put a basket on your bike, and find a bag that fits it. There are some bags commercially made for such purposes, but usually any small messenger, duffel, briefcase or backpack will do fine. The photo shows my own setup for a day of substitute teaching: a front basket with an iPad-sized messenger bag to hold a book, my tablet and my lunch.