Wald folding baskets along both sides are great. Sure, they're a bit heavy and you can't remove them for a little joy ride, but the hold the weight down low, hold a ton of stuff, you can fold out one or both depending on your needs, then you can just throw a backpack or satchel or whatever in them, string a bungee cargo net over the top and off you go. Low theft risk as they're bolted on, but your bag goes with you. They're relatively inexpensive (more than a plastic milk crate swiped from a back alley, less than a nice set of panniers). Some would say they make your bike look cheaper (I prefer to say they make it look more useful), but, if it does make it look a bit more low end that must means it's less likely to be the target when parked. My two safety goals when parking are to have a bike that looks A) less desirable than the bikes around me and B) harder to steal than the bikes around me. No security system is perfect, but that works better than nothing.