A - Adjacent apartments
the girls and the boys lived right across the corridor throughout most of the 10 series until they started swapping places i.e chandler and rachel swapped
B - Births
a few of these:
carol gave birth to ross' first kid ben
Phoebe gave birth to her brothers triplets: chandler, frank jnr jnr and lesley
rachel gave birth to ross' second kid emma
erica gave birth to monica and chandler's adoptive twins jack and erica
C - Central Perk
this cafe was the friends local and was shown in most episodes if not everyone