It may be persuasively argued that the foundation for what is called the "Golden Age of Athenian Democracy" was laid when the conservative leader of the Aristocratic faction, Kimon, was exiled. Kimon had been a much-admired military hero, but his popularity had utterly collapsed when he had persuaded the Athenians to send military aid to Sparta in 462 BC-- Sparta was facing a major helot uprising at the time. Kimon had always been an admirer of Sparta, and even though Kimon's rival Ephialtes maintained that Sparta was a deadly rival and should be left to fend for itself, Kimon managed to get his way. The expedition ended in total humiliation: the Spartans, wary of Athens, expelled Kimon and his army!
Kimon was seen as having shamed Athens and himself with his embarrassing failure, and in 461 BC, he was ostracised from the polis. The reformer Ephialtes then had an unprecedented chance to seize power. With the support of Perikles, he first reduced the power of the Areios Pagos (a.k.a. 'Areopagus'; a high court that served as a council of elders, and which was the great bastion of the conservative aristocracy). Power was transferred to the Council of Five Hundred, the Assembly, and the popular law courts. Other policies favoured by Kimon were also reversed, including his pro-Spartan policy. From that point on, the animosity between Athens and Sparta would only build and build-- culminating in the First Peloponnesian War (460–445 BC) and the (second) Peloponnesian War (431–404 BC).
The way Kimon's expedition turned out, it seems, had major effects! The question I'm asking here is: what if Kimon's Spartan expedition is a resounding success? Let's say he is in somewhat closer contact with the Spartan leadership, and they let him prove that he's truly on their side. Which he was... so they let him stay and support them. After the Spartan victory over the Helots, Kimon returns to Athens, once more confirmed as a great military leader and a true hero. His political fortunes look a lot better than in OTL.
Also, Ephialtes was murdered in 460 BC in OTL (and the most popular theory is that it was by a member of his own faction, since if it had been one of his conservative enemies, Perikles would certainly have used the event to turn Ephialtes into a martyr-- which he did not.)
So let's suppose that either Ephialtes still gets killed around the OTL time, and Kimon uses his increased political clout and the weakness of the opposite faction to exile Perikles. Or that he exiles Ephialtes, and later manages to sideline Perikles anyway. Neither seems unlikely to me. Kimon was actually a crafty operator; he had no way of knowing the Spartans - with whom he'd always enjoyed such good relations - would so crudely rebuff him. He was not, as some have asserted, some kind of rash fool. So if hew retains his status and popularity, I can see him getting rid of his rivals.
This would prevent the democratic reforms of Ephialtes and Perikles. With Kimon and the aristocrats firmly in charge, the 'Golden Age of Athenian Democracy' gets delayed at the very least. Quite possibly it gets prevented outright. In OTL, ephialtes acted at just the right time, having gotten a once-in-a-lifetime chance to exploit Kimon's fall from grace. Without that random chance, there may never come an opportunity to suddenly implement radical reforms. I'm fairly sure some reform of the aristocratic system could and would be forced, over time... but Athens might never become the more democratic polis with which we are all so familiar.
A lasting Aristocratic Athens. Is that a realistic option here? And do Kimon's good relations with Sparta have major effects? His ATL success in Sparta would get him goodwill there, and would likely convince Athens to get fully on board with his pro-Spartan policy. So might the Peloponnesian War even be prevented? Athens and Sparta are still rivals, which is natural and almost inevitable... but might it be reduced to a rivalry that does not result in such utterly destructive wars?
If this is a realistic perspective... what might be the further ramifications of such an ATL?