At least in our class we hardly even talked about that prong - the focus was definitely on activities substantially affecting interstate commerce. For instrumentalities, though, it's just like everything else in law school - you can make arguments on both sides. I'm guessing the decision hasn't been hashed out enough to say conclusively what constitutes an instrumentality and what doesn't. So on the one side you'd argue that the whole point of Lopez was to limit the ridiculous extent that the Wickard v. Filburn test was taken too and thus instrumentalities should be construed narrowly. On the other hand, in Gonzalez you see the court going back to expanding the commerce power and thus could argue that in its current state the court would construe the prong broadly. In any event, I'm guessing that Congress could definitely not regulate any activity of anyone who has ever crossed a state line. I'm also guessing they certainly could regulate truck drivers who transport goods across state lines (the other example given). In between those two there's a lot of gray area.