Some fantasy fiction and FRPGs include the concept of mana, the fuel that powers magic. Some legendary locales have high mana, allowing more spells to be cast. Some computer games have mana potions (potions of energy, in Nethack.)
In FRPGs, mana is usually tied to spell point systems. D&D doesn't include mana by default. But if you want to add mana to D&D without switching to spell points, here's a simple way: mana is rated 1 to 6 (or 1 to 9, whatever your maximum spell level is.) A locale with high mana allows spells of the given mana level or below to be cast without being forgotten, so a Mana Level 1 locale grants unlimited 1st level spell casting, but only for spells that have already been prepared.
These locales should be rare, spoken of in rumor or legend, and should be limited in an additional way (only at dawn or dusk, or only when a libation of wine is poured on the ground, for example.) The higher the mana level, the rarer the locale, and the more extreme the requirement (only during the new moon, only on Midsummer's Day.)
Mana potions are also rated with mana levels (probably no higher than 3) and should be very rare; they require some kind of liquid collected at a high mana locale to brew. The effects allow unlimited spell casting of the low level spells for 1 or 2 hours.