Unripe Mandarins

I would use recipes that call for calamondins (calamansi) or sour oranges, perhaps adding a bit of lemon juice to make them even a little more sour.

I had this happen as well and juiced them. Enjoyed it like orange juice!

Marmalade! If they are sour they will make even better marmalade than if they are sweet.

Thanks so much for the great suggestions, I completely went blank with what to do with them. Happy I can get some use from them.

Slice & Dry them? Would make a good tea. Or at the least potpourri.

Citrus won't ripen off the tree, nor will it develop sugars without some cold weather. Sour mandarin juice can be a good addition in marinades, barbecue sauce, chilis etc., much as you might use lemon or lime, though the flavor won't be as strong.

you could juice them and use in a salad dressing , sweeten as Nancy suggested.

I'd be afraid to add them to expensive other ingredients in case their unripeness ruined the dish or drink.
Instead cut them up, mix with some sweetener (for fruit salad) or salt (for savory salad). Cover and leave at room temp overnighter half a day, to see if the seasoning helped.
If yes, proceed.
If not, consider them sunk costs and discard.