Question: Is there any way to have a [Decanter of Endless Water](https://www.dndbeyond.com/magic-items/decanter-of-endless-water) pour out other drinkable liquids? If so, how could I get one? It seems reasonable to me as a possibility. If not how might you go about modifying a decanter of endless water to do so?

Answer: 
The less-than-serious scenario pack for AD&D 1e *Castle Greyhawk* contains a Decanter of Endless Lemonade. And many other silly things.
The Alchemy Jug (as mentioned above) would be closest, but also note that Hoard of the Dragon Queen p.74 contains a Tankard of Plenty.

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> The Golden Tankard takes its name from a magic item that Raggnar found years ago: a golden stein decorated with dancing dwarves and grain patterns. This is a tankard of plenty. Speaking the command word (“Illefarn”) while grasping the handle fills the tankard with three pints of rich dwarven ale. This power can be used up to three times per day.
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For lore/historical reference, this is similar to the Everfull Mug from 3.5e's Magic Item Compendium, p.160, which was possibly related to AD&D's Mug of Plenty (Encyclopedia Magica).