He cannot create laws, he can only approve laws and budgets that are submitted by Congress. He cannot decide on whether laws or actions are Constitutional or not - this is the Supreme Court's job.
There are many powers the president does not have. In fact most powers he does hold have had to be legislated for. He cannot revoke the Constitution for instance nor serve more than two terms in...
The president has the power to send in U.S. troops but only congress can declare war. Some other executive duties are foreign relations and arranging treaties. The president chooses the department...
Separation of powers is the power of each branch of Government which although are separate are not entirely separate as each branch provides checks and balances for the others