I see no reason to rent a car. You can use bus and tram to move around. Astir beach is at the edge of Glyfada towards Voula unless you mean an other beach further out? Golden Sun is near by, again on the busy Posidonos avenue if I'm right. Glyfada is famous for its high quality shopping, although it's mostly about brand names and expensive cloths, if you are looking for bargains you have to look elsewhere. Please note though, that Athens city center is a 40 minutes or more tram ride. A 45 minutes bus ride to Piraeus port- depends on traffic of course... If you mostly want to look on the sites, you should better stay in Athens center, thean move to the beach for a few hours. If you mostly look for the beach, maybe look further out of Athens on locations such as Voula,Vouliagmeni,Kavouri etc. If you want to compromise both and limit your traveling time, maybe look on Paleo Faliro, Alimos or something, where beach is very average-if any- but you get the sea and you are much nearer Athens center.