I put skelatal racks on most of my bikes. They add ounces and are there if need be ...
+1 for the trailer idea. If it's serious shopping and semi-decent weather, just pull the trailer. Get home, un-hook and you have a naked bike
Although I also agree on rescuing a toss-away bike to make an errand bike out of. It's saving one from the landfill/crusher and it keeps the fun bike, fun