If it were me, I'd also be looking at scenarios that involve any kind of "thing" that I could acquire cheaply and re-purpose somehow. Making lamps out of old wine bottles, that sort of thing.
Robots, unmanned vehicles of various sorts, all sorts of small electronic gadgets, probably some auto accessories... really, the range of things you could (at least hypothetically) manufacture in a space that size is huge.
Now whether or not you could manufacture the thing at scale may be a different question. You could probably easily accommodate doing something the size of a small home appliance (think: washing machine size) if you only had to do one at a time. But doing that at scale might well require more space. So is the intent to stay in the garage and run an enduring business there, or just to ship a prototype, prove the model, and then maybe expand? Or is this purely an academic thought experiment?