Most of the added content is for later in the game, I'd suggest killing the Act 1-5 bosses to get Fragments.
Also killing Countess/Summoner And Nihlathak to get Keys is a good way to start off as well.
Or even doing Hell Cows over and over can get you some highrunes if you keep at it long enough.
As for the easiest custom endgame content, I'd say Plaguelands and the Pits in act 5 are the easiest.
Anything other than those two places is a bit too hard for a starting character.
There could be a few other places you might have some luck in like the Swampy Pit in act 3.
Or Inferno Cows, but getting passed Ry'leh and Azathoth isn't exactly a piece of cake.