I use a retired kid trailer with no modifications. It's a Bell two-seater, with lots of pockets that hold things like loaves of bread perfectly. I have hauled a regular load ( think two adults, two large teen boys) many times, complete with gallons of milk, sacks of potatoes and eggs.

One time I rode to the specialty butcher and got a very nice prime rib roast and picked up a cake from the baker's - the seat belts work great for holding things in.
My usual run is grocery, video store, library which about 2 miles each way. If I add in the post office, bank and lbs it turns into a modertely hilly 7 mile loop. I use either my geared commuter (converted old mtb) or my singlespeed.

Funniesst moment. OK, I get "the look" as in "omg she's got kids in traffic" look a LOT but one time I lost my hitch in the middle of a somewhat busy interserction. Potatoes were rolling everywhere. Several latte-suv moms stopped completely freaking out that somehow my kid was careening through the intersection and I am having to calm people down... it's only groceries, folks... move along now....