Congress was given the power to regulate commerce "among the several states". This has been interpreted by the USSC to mean interstate commerce into the state. Although Congress has this power, it doesn't mean they must use this power.
I hope we're in agreement up to this point.
For the first 100 years of our country's existence, there was very little "interstate" commerce; most commerce was local. As interstate commerce increased, it was found that local, intrastate commerce could have an effect, in some cases a substantial effect, on the interstate commerce that Congress was attempting to regulate.
One early example that I found was the 1914 Shreveport Rate Case. Congress was regulating interstate rail shipping charges (it is irrelevant why or if this was a good idea). States were complaining that intrastate rail rates were kept artificially low and were being offset by higher interstate rates -- that, in effect, outside states were subsidizing local traffic. The USSC ruled, "It is for Congress to supply the needed correction where the relation between intrastate and interstate rates presents the evil to be corrected, and this it may do completely, by reason of its control over the interstate carrier in all matters having such a close and substantial relation to interstate commerce that it is necessary or appropriate to exercise the control for the effective government of that commerce."
But, the court added that Congress had no authority to regulate purely intrastate commerce: "Congress thus defined the scope of its regulation, and provided that it was not to extend to purely intrastate traffic. It did not undertake to authorize the Commission to prescribe intrastate rates, and thus to establish a unified control by the exercise of the rate-making power over both descriptions of traffic."
The judge made one other point, which really sums up my understanding of the commerce clause as it relates to state activity: "It is unnecessary to repeat what has frequently been said by this court with respect to the complete and paramount character of the power confided to Congress to regulate commerce among the several states. It is of the essence of this power that, where it exists, it dominates. Interstate trade was not left to be destroyed or impeded by the rivalries of local government. The purpose was to make impossible the recurrence of the evils which had overwhelmed the Confederation, and to provide the necessary basis of national unity by insuring 'uniformity of regulation against conflicting and discriminating state legislation'."
So, to answer your question (finally). Congress has no power over any intrastate activity (commerce) that does not or may not have a substantial effect on the interstate activity (commerce) that Congress is attempting to regulate. There were two recent USSC cases where it was ruled that Congress over-reached their use of the Commerce Clause: The Violence Against Women Act (VAWA) and the Gun-Free Schools Act (Lopez) were both ruled unconstitutional.
Congress may be regulating something you think they shouldn't be regulating. Fine. But that doesn't make it unconstitutional. Every two years we elect the people who write the laws. That is where we have the most impact, not sitting at our keyboards saying that the USSC is out of control.
The judge made one other point, which really sums up my understanding of the commerce clause as it relates to state activity: "It is unnecessary to repeat what has frequently been said by this court with respect to the complete and paramount character of the power confided to Congress to regulate commerce among the several states. It is of the essence of this power that, where it exists, it dominates. Interstate trade was not left to be destroyed or impeded by the rivalries of local government. The purpose was to make impossible the recurrence of the evils which had overwhelmed the Confederation, and to provide the necessary basis of national unity by insuring 'uniformity of regulation against conflicting and discriminating state legislation'."
Apparently, it is relevant as to why, and wheather it was a good idea.
Actually, we're not. The common meaning of the word "regulate", circa 1780 was "to keep in good working order". "To regulate commerce among the several states" was as much an establishment of responsibility as an enumeration of power.