Both ice cream and soda water would technically be available in a D&D setting. Both of those were developed in the mid 18th century, and could appear in similar manners to how they do today.
Ice can easily be made available anywhere it freezes in winter and there's enough industry to cut and store ice from frozen lakes. Packed it sawdust, an icehouse will keep ice through summer in such climates allowing ice to be available year round, if not cheaply. Ice cream requires access to a hand ice cream maker. This consists of a barrel or bucket packed with ice and salt with a metal bucket placed inside it where the mixing will be done. A wide paddle or stir is placed in the metal bucket that can scrape (or almost scrape) the edges of the bucket, and the sugar, cream, egg yolks, and flavorings of the ice cream "batter" are poured in to the bucket and the paddle stirred. Having done it myself growing up, it's
very labor intensive to do, but it does work, and it's only moderately more difficult than creating a butter churn. The best ice cream makers had a geared paddle stir to make it easier, though. The ice and sugar would probably be the most expensive parts.
A method of easily creating soda water was discovered by Joseph Priestley by, essentially, simply mixing water and carbon dioxide (then, "fixed air"). You can still find his paper,
Impregnating Water with Fixed Air, where he uses sulfuric acid and chalk to create carbon dioxide and then mixes the gas with water. If you read his introduction you'll see how he's trying to sell soda water as safer alternative for the navy for medical reasons!
He summarizes the issue well:
IF water be only in contact with fixed air, it will begin to imbibe it, but the mixture is greatly accelerated by agitation, which is continually bringing fresh particles of air and water into contact. All that is necessary, therefore, to make this process expeditious and effectual, is first to procure a sufficient quantity of this fixed air, and then to contrive a method by which the air and water may be strongly agitated in the same vessel, without any danger of admitting the common air to them; and this is easily done by first filling any vessel with water, and introducing the fixed air to it, while it stands inverted in another vessel of water.
While I find soda water is technically possible to create, I don't think it's that feasible in a D&D world because of the lack of chemistry and theory of gasses and elements. Creating carbon dioxide from acid and chalk makes sense to 18th century scientists, but probably not so much to Wizards and Sorcerers who have a more
Aristotelian view of elements. Perhaps some gnomish artifacers could come up with a contraption for doing it, however. Once you've got soda water, it's just a matter of adding sugar and flavorings, filling bottles, and corking them like wine.
