Shiners. They're big pretty minnows, and at least one species is present in just about every state in the US. They're fine in goldfish temps, they won't threaten your goldfish, they're active, and they're rather pretty. I particularly like blacktail shiners- they almost look like silver trout, but smaller.
Also, some sunfish might make suitable tankmates. Some sunnies can be a bit aggressive, but a warmouth sunfish should work, they don't tend to be mean. They just have really big mouths, so I'd be a bit cautious keeping them with anything small.