The fun is in the details, but I thought Walter Dellinger gave a cogent Con Law for Dummies in Slate:

Constitutional law: the five-minute crash course

The basic rule of American constitutionalism is this: Before the government can forbid you from doing anything, it has to provide a reason. "Because we say so" does not count as a reason. To limit ordinary liberties (like selling eyeglasses), most any reason is good enough. To restrict fundamental liberties (like using birth control while having sex), however, the government must have a really important reason. (Getting to decide which liberties are "fundamental" is one of the cooler parts of being a justice.) Under the equal protection clause, even if the government has a plausible reason for putting a burden on you, it also has to explain why it treats other people better. If the justices suspect that the government may simply dislike people like you, they will demand an especially convincing explanation for the different treatment. And if the government wants to interfere with your liberty by actually taking your house or property, it has to pay you "just compensation"; and even if it's ready to fork over compensation, the town can't take your stuff at all unless it's going to use it for a "public purpose." (Current hot topic: Is forcibly taking old people's homes to make way for a spiffy new Wal-Mart really a "public" purpose?)

Establishment of religion is really simple: government prayer, bad; private prayer, good. (The only hard cases come when a citizen uses government property or public funds for religious purposes, and the facts make it difficult to tell whether it's the government or the private citizen actually making the religious choice.) As for speech, you can generally say whatever you want, but not necessarily where, when, or how you want. It's also OK for the government to regulate "expressive conduct," as long as the government is going after the "conduct" part and not the "expressive" part. Also, you have no right to dance naked unless you are a really, really good dancer, in which case it becomes art. As for the First Amendment's so-called "reporter's privilege" to protect confidential sources, you can skip that—it's taught with reverence in journalism schools, but judges never heard of it.

Congress has the power to ban anything from crossing a state line for any reason. And Congress can regulate any activity that's economic even if it's wholly within one state. But if it's not economic, and it doesn't cross state lines, then Congress cannot regulate it—unless five justices think the regulation in question is a really, really good one. This national power is limited to some extent by state sovereignty, a doctrine traditionally invoked by those on the right to insulate conservative red-state practices from federal regulation. This doctrine is now eyed fondly (but warily) by liberals seeking to protect blue state positions on gay marriage and medical marijuana. Rule of thumb: State sovereignty claims are more likely to be upheld by the current court when advanced by Alabama than when put forward by Oregon.

Finally, one needs to understand judicial restraint, the doctrine that a judge should avoid "legislating from the bench" and should instead strictly apply the text of the Constitution "exactly as written." This approach is very appealing to those who have never read the Constitution. But see, e.g., the 14th Amendment, which requires judges to protect (without any further elaboration) "privileges or immunities of citizenship" and "liberty" and "equal protection"—phrases so open-textured that they make the "apply exactly as written" mandate somewhat unhelpful.

Senators especially like it when a nominee says a judge's role is just to be an "umpire." But broad constitutional phrases are different from sports rules, so a judge would be like an umpire only if the game—instead of having a strike zone and a set number of balls, strikes, and outs—provided instead that "each batter shall have a fair chance to hit the ball" and "each team shall have a reasonably equal opportunity to score runs." Key language of the Constitution is that broad, meaning that men and women appointed to the bench must necessarily exercise judgment. Which is, of course, why they are called judges, and not umpires.

Seriously, that captures the gist of Constitutional law. But if you read the consititution you wouldn't recognize most of what he just said. Its a very simple document and the barnalces of interpretation have buried its simplicity.