I am a freshman in college and I want to spend my free time reading different works and expanding my ideas. I don't necessarily want to start linearly (from Classical Greece and on), but if that is the best way, I'd be open to it. So far I've read a little bit of Camus and Descartes, both of which I've enjoyed. I would like to start off with something more easily digestible, but a little bit of rigor goes along way in the learning process.
Some people you could read without having read from the beginning. Nietzsche, Descartes, Sartre. But if you're wanting to tackle guys like Berkeley, Heidegger, Kant or even newer guys like Saul Kripke you're going to want to go back and read Plato and read the guys from the analytic tradition like Bertrand Russle, Frege (even if he was kinda off about his ideas), Leibniz etc.
The problem is you're going encounter philosophers who use words like "idealism", "monad" and other words that may or may not seem familiar and often have different meanings than what you're used to them meaning. Reading, or at least having a good understanding of, the works these guys read to come up with those words is important.
If you're interested in philosophy I strongly recommend checking out the Partially Examined Life podcast. It's very informal, but very interesting discussions.