I’d probably squeeze the juice and freeze it. Then use the skins for chimia (a type of jelly - see further info below*). Here’s the recipe:

• 3 cups of grape skins. Don’t include seeds, they get bitter.
• 2 cups of water.
• 1/2 cup of sugar.
• 1/2 cup of lemon or lime juice.

1. Cook the skins in the water. Let it boil for ten minutes or so, on low fire, then drain the excess water.
2. Add sugar and lime juice. Keep cooking it on low fire and stirring it. The skins should fall apart on their own and thicken the jelly, but if you want use a blender to speed up the process. Keep in mind that the final result will be thicker when cold, so don’t cook it too thick.

That’s it. If preserving it put it inside pots while still boiling hot, and they should outlast the thermal death of the Universe.

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*further info: @[email protected] mentioned that the distinction between jam and jelly depends on the country for English speakers. Well… when you speak Portuguese it varies regionally in Brazil (and likely in Portugal, too), and it might have one to three categories. I grew up with three:

• geléia or geleia - jelly made with whole fruits, either heavy on pectin by themselves or with added pectin (e.g. from the white part of lemons). It sets hard, with a gelatinous consistency.
• doce - the word means literally “sweet”, and it’s used for stuff like dulce de leche (doce de leite) or desserts, but when it comes to fruits it’s usually “jam”. No pectin added, so it’s usually runnier
• chimia - at least I see it as a type of geléia/jelly, but a lot of people see it as a third thing, aside from the other two. It’s traditionally made with pomace, as a way to reuse leftover skins from wine production; because otherwise the drunkards would make graspa aka bagaceira (grape pomace spirit) out of it.

That’s basically as far north as Paraná though. Norther than that (São Paulo) and people don’t use the word “chimia”; go further north and they take “geléia” and “doce de frutas” as synonymous.