I'd be making jelly or wine like there was no tomorrow. Plum wine in special is delicious.
There's a recipe here, if you want; it works for fruits in general, not just grapes. Note: most equipment can be improvised with some redneck engineering, for example my water lock is simply a piece of flexible tube forced into a U shape with an old hair rubber band, and filled with water.
You could also share the plums with neighbours. Bonus points if you manage to barter them for something else.