Wash them and cut them off the pits.
Dehydrate them to chewy sweet jewels to snack on.
Fruit leather: plums, sugar, blend fine, dehydrate on a mesh covered in plastic wrap, roll up and store (fridge or freezer).
To find a good plum butter --make them like apple butter but use plums. I hear it works best in a slow cooker so they don't burn or scorch. Once done (many hours of slow cooking), sanitize jars lids and screw lids, pour into jars, top with lids and water bath can for 30 minutes, cool and store. OR put in jars and freeze (any type jars--leave some headroom for expansion--mom used to do this).
Plum meat sauce: plums sugar salt vinegar cinnamon cloves--cook down to a sauce, use a hand blender to make it homogenous. (like a ketchup only spicier) (fridge or freezer)
I miss my plum tree.