I can't think of any history class I ever took where the period in Greek history between the death of Alexander and the Roman conquest was covered. All courses on the Greeks seem to always end right after Alexander dies and a brief overview of the successor states (generally just the Seleucid and Ptolemaic Empire).
The next time you hear about them is always that they got conquered by Rome, which occurs roughly 200 years later. Was there anything interesting going on in Greece at this point? Was it just a period of a long and slow decline?
Additional, not totally related question: In high school I had a teacher who seemed to absolutely love the Spartans. I can't remember if he was talking about the Macedonians or the Romans, but he said that instead of try and conquer Sparta they just conquered everything around them and forced them to subjugate due to their isolation. None of my Professors ever mentioned this. Were the Spartans so fearsome that they were actually just avoided by the conquerors?
The Hellenistic period is the era between the death of Alexander the Great and when the Romans took Egypt (323 BC to ~30 BC). Quite a lot happened during this period. I couldn't possibly cover it all in depth within a reasonable amount of time, so I'll keep it short and to the point.
Mainland Greece began to decline in power and would not become a major power until the modern era. Greek wealth and culture moved towards Egypt; many people did recognize this and began to move to where there were better economic opportunities. As a result, mainland Greece saw an evident decline in population.
Formerly powerful city-states became Macedonian puppet states. Athens was besieged and fell to Macedon in 262 BC. They convinced Macedon to leave around 20 years after this happened by getting rid of their own navy and walls; Macedon left because Athens got rid of its own power and was no longer a threat.
Sparta was a bit more reluctant than Athens to give into Hellenistic rule. They still had massive population decline which meant that wealth and power was concentrated into few hands; by some accounts Sparta had 400 citizens left at one point in time. King Cleomenes came to power in 229 BC and knew Sparta had to be repopulated somehow; he exiled the richest Spartans, seized their wealth and redistributed it between a 4000 new Spartan citizens. He also reinstated the agoge (which had been lost as Sparta's population declined) and trained them in the Macedonian phalanx, soon after defeating the Archean League.
Were the Spartans so fearsome that they were actually just avoided by the conquerors?
That's a good segue to your other question. At this point it wasn't so much that they were fearsome; it was just that they weren't worth paying attention to as they weren't an imminent threat. Macedon took notice of them after Cleomenes began instating these new policies - presumably because they considered Sparta to be an up-and-coming threat - and conquered them; that was it for Sparta.
Now onto Egypt. After Marc Antony and Octavian defeated the conspirators that killed Caesar in the Battle of Philippi, the two began to butt heads; this was a clash over who would hold power in Rome. Antony recruited the help of Cleopatra against his new enemy Octavian; the former two stationed themselves in Egypt while Octavian was stationed in Rome. In the ensuing battle between the two factions (31 BC) off the coast of Greece Cleopatra realized they were outnumbered and she fled back to Egypt; Antony heard a rumor that she killed herself (or captured by Roman forces depending on which account you're reading), he killed himself, and eventually she actually killed herself in 30 BC. When Cleopatra died Egypt went to Rome, and this was the end of the Hellenistic Period.
Source: "The Greeks: History, Culture, and Society" by Ian Morris