There is an express bus that runs all night and it's perfectly safe to catch. The bus departs from Syntagma Square, so it makes sense to book a hotel near there.
You can confirm timings and cost nearer the time, but the bus is very regular and currently costs 3.80 euros.
Hey again aneckc:-) Got you on your other post..and if it hasn't changed, it's currently 3.20E per person..but who knows by the next year! Enjoy!