Unfortunately, nothing comes to mind given the combination of time of the year and time of the day (if "soon" means within the next 10 days or so). The sun sets around 6 p.m., preventing you from visiting archaeological sites in Athens or a nearby beach. Moreover, the evening rush hour would make you nervous unless you take a train/metro. By the way, to get to the centre of Athens you'll need about 45-60 minutes, one way, which leaves about a maximum of 1-2 hours to walk around the centre of the city. The only possibility (although too early in the day for that) I can think of is to take a cab to Rafina, have an early seaside dinner at a taverna (weather allowing) and then back. Bottom line: I'd suggest to "arm" yourself with some good reading, one or two crossword puzzles and... patience!