Plums are easily frozen. They also make a lovely liquor... usually called Plum Gin. I usually use vodka or Everclear as a base. In a large glass/stoneware crock, slice a bunch of plums and put in a good amount of sugar (look on line at recipes). Then you stick in a dark corner, shake every couple of days for a month or two, and let the plum juices leach out into the alcohol until the sugar melts and it becomes a glorious color. The soaked plums are delicious!
Mangos I would probably cube or puree and process as ice cubes. I've never had the problem of too many mangoes! I might process some down into a filling, adding sugar/cornstarch to add into baked goods.
One of the things I'm doing at this time of year is adding the slightly crunchy mangos (but fully ripe is always to die for) to my black bean salad/salsa. Start with a can of black beans, rinsed. I linked a starter recipe over in this thread:
Corn Dumplings in Coconut Corn Broth