There's been quite a lot of planar adventures in Dungeon over the last few years; here's the high level ones though... (warning: Minor spoilers below for Shackled City and Savage Tide adventure paths):
#108: The Iron Satyr (11th level, Acheron)
#116: Asylum (19th level, Carceri; last adventure in the Shackled City Adventure Path)
#123: Quicksilver Hourglass (30th level, unique demiplane)
#136: Gates of Oblivion (18th level, plane of Shadow)
#141: Vlindarian's Vault (18th level, Elemental Plane of Fire)
#144: Diplomacy (18th level, unique demiplane)
#147: Into the Maw (17th level, Abyss [Gaping Maw], part of Savage Tide Adventure Path)
#148: Wells of Darkness (18th level, Abyss [Pazunia, Wells of Darkness], part of Savage Tide Adventure Path)
#149: Enemies of my Enemy (19th level, Abyss [Shendilavri, Pazunia, Thanatos, Blood Shallows], Hades, River Styx, the eladrin Court of Stars), part of Savage Tide Adventure Path)
#150: Prince of Demons (20th level, Abyss [Gaping Maw], part of Savage Tide Adventure Path)