You may want to take some time to study your U.S. History yourself actually. The Constitution enumerates various powers and responsibilities of the various branches of government, it is not however an exhaustive list. The enumeration of powers was precisely why a number of the founding fathers believed that the Bill of Rights was necessary to provide protections for, and enumerate various essential rights of the citizens. The Constitution sans Bill of Rights, for example, would not guarantee freedom of religion or speech, these are not inherent in the body proper of the constitution.
As for "legislating from the bench", usually that is code for "made a ruling I disagree with". Judges can and in fact MUST interpret laws and apply them, and if you think judges are sitting there coming up with meaning out of nowhere just because they feel like it its clear you have never spent anytime in the legal system. There is far more to the law than what the judge feels like, a legal opinion is built upon precedent from previous cases and the general body of law beyond just the constitution. Laws, much like the constitution are seldom exhaustive and complete, how could they possibly anticipate all possible combinations and confluences of events? So there is a cycle. Congress creates a law, the President enacts the law, the Courts interpret the law. If there is disagreement with the interpretation there are two (non exclusionary) paths to take. One is to appeal the case to a higher court, the other is to pass a law which addresses the disagreement and clarifies any disagreement. Ultimately its even possible to amend the constitution to provide a new basis from which a law can be built.
The "Founders" were not some single monolithic entity with a single intent, there were a group of men of a variety of perspectives who argued, sometimes vehemently over the best approach to take, Federalists, Anti-Federlists, etc. They didn't HAVE one unified vision for the Constitution or the future of this country beyond broad strokes of well, not being British anymore. The Constitution + Bill of Rights was a compromise document incorporating the views of this diverse group. To act as if there was as single purpose and philosophy behind it is simply inaccurate.