Julia Annas has produced a remarkable volume intended as a reader for introductory ancient philosophy classes. I'm using it in my class right now, and I'm finding it to be exactly what I was looking for.
An upper level ancient philosophy course should be more directed toward examining the whole of a philosopher's thought, and reading longer works in context with the entire philosopher's outlook is ideal in that environment. In an introductory course, however, students are taking philosophy for the first time, and the ancient philosophers are merely a means to learning philosophy for the first time. Focusing on issues is thus more important than getting the whole of a philosopher's thought down in every way.
This book presents six topics, with ancient philosophers' writings on the topics organized as a conversation. The six topics are (1) Fate and Freedom (which includes divine foreknowledge and the fixity of the future), (2) Reason and Emotion, (3) Knowledge, Belief, and Skepticism (including relativism), (4) Metaphysical Questions (including paradoxes, the Forms, cause/explanation, and time), (5) How Should You Live?, and (6) Society and the State.