Enjoying a great climate year round, Athens is worth visiting at any time and offers some serious sightseeing opportunities.
There's a fantastic range of museums and galleries around the city dedicated to Greek history and culture as well as Islamic art. The Roman Agora is the best known agora (public meeting place) in the world. It contains the famous Tower of the Winds, a clock tower build around 50BC with sun dials, a wind vane and water clock.
For great places to eat and shop, the areas of Plaka and Anafiotika offer lovely surroundings and world-class restaurants, tavernas and shops. It's also well worth visiting the cathedral, Lykavittos Hill (the highest point of Athens), the Panathenaic Stadium, which hosted the first Olympics of modern times in 1896 and, of course, the temple of Olympion Zeus, once the largest temple in Greece and now preserved as beautiful iconic ruins.
The best months to visit Greece are perhaps July and August, when the temperatures are beautifully warm, but June and September are still warm and have fewer crowds.
Car hire in Athens is quite convenient. You can book and pick up cars at Athens Eleftherios Venizelos Airport (ATH) as you arrive in the city. Athens is beautiful to drive around, with its lovely architecture, winding roads and places of interest all around the city. There are car parks in various convenient locations for visiting the popular tourist spots.