Congress can only regulate activities under the Commerce Clause.  This term is regularly used in Commerce Clause jurisprudence.

What does activities mean?  Could it mean inactivity?

Here’s the Webster’s Dictionary definition of activity:

1) the quality or state of being active; action
2) energetic action; liveliness; alertness

There are a few more examples as well, but all clearly indicate affirmative actions on behalf of someone.  Unless the Supreme Court decides it will reverse all of its opinions that state the Court must examine the activities being regulated, the inactivity of not buying health care shouldn’t be subject to regulation under the Commerce Clause.  To put it another way, one doesn’t affirmatively or actively take a step not to so something.