what to do with 15+ lbs. unripe plums?
Out of some of them you could try making green tkemali (Georgian sour plum sauce), which is very good with grilled meats (esp. shashlik) and roast chicken.
For ~2 lbs unripe (sour) plums: simmer with water to cover until soft, about 10 mins (time varies w/size of plums). Drain, pit and put through strainer or food mill, and return puree to saucepan. Mix in 1 tsp chopped garlic, 1/4 cup each chopped fresh cilantro and dill, 1/8 tsp hot paprika and ~2 Tbsp lemon juice (more or less to taste, depends on how tart plums are). Salt to taste, bring to boil and simmer 2 minutes; cool and store in bottles in fridge.