I dehydrated many into prunes.
Others I made into juice using my handy-dandy Breville Juicer.
Plum juice is often very sweet so we usually mix it with apple juice.
The leftover pulp from most fruit can be made into jam, or sauce or desserts. The pulp from apples is usually full of seeds and core bits so I use it for compost or to feed the chickens.
The pulp can also be frozen in small containers to use at a later date.
Here is the start of a Plum Crisp using the plum pulp. I greased a 9x9 pan and filled the pan half full of pulp mixed with about 1/2 cup sugar (depends how ripe your plums are) and a dash of cinnamon.
For the topping - mix 1/2 cup butter with 1/2 cup sugar or brown sugar, add 3/4 cup each of flour and rolled oats and mix until crumbly. Sprinkle on top of the plums and bake for an hour at 350 or until bubbly around the edges.
Enjoy warm with ice cream.