Try to visit the Acropolis at any time but summer, but if you are there in summer, start out early in the morning, as climbing it gets very, very hot. You don't need a tour group. Start at the Acropolis Museum, below, which contains many of the artifacts that are not in other world museums; however, I found it somewhat boring and empty, though the architecture is superb. Then head up to the Acropolis. Consider that the whole Parthenon was painted 2500 years ago, and an enormous status of Athena stood in its center. You have great views of both the Theatre of Dionysus, where some of the great classical plays of all time were performed, and the Odeion of Herodes Atticus. Look the other way and you'll see the Greek agora or marketplace, where philosophers and politicians discussed ideas. If you approach by ground from the opposite direction, you'll first enter the Roman forum before getting into the Greek agora. When I was in the latter it was a bank holiday (I couldn't help an ironic chuckle) so there was no charge, and no one but me there. It gives you some of the best views of the Acropolis.