A plane consisting entirely of intellectuals and artists: wizards, philosophers, poets, musicians, etc. Just a paradise of learning and culture. The plane is mildly positive energy-aligned and timeless, keeping all beings therein eternally youthful and precluding the need for resources (e.g., food is only taken for pleasure, never for sustenance).
Valhalla. Carouse in a mead hall, then go kill some (<=4HD) giants.
The ultimate sensory deprivation chamber. No gravity (so no interoception); no medium capable of transmitting sound or visual phenomena.
Planes dedicated to the pure, distilled essence of single experiences. For example, a plane of reticulate twisting hallways where one hears whispers of ultimate knowledge, always hiding just behind the next corner; a plane where one is continuously born; a plane where every sentient creature continuously falls in love with whomever it sees, weeping in perpetual joy.