20-38 Scarlet monastery.(cathedral)
38-45 Zul’ferrak (35 to enter,but 38 is a good level to start)
45-50 Scholomance,Straholme (Still leveling)

50-59 Alterac valley Battle grounds.

59-60 Questing and some ramparts dungeon.

60-68 Outland questing.

69+ Northrend questing.