As far as "what else" you can do with wild plums, you can make your own plum liqueur with them.
Again, lots of recipes out on the web, but here's what I have done with them:
Take 2 quarts of washed plums and place in a 2 quart canning jar. Breaking the skin on them really helps. Add one cup of sugar. Fill the remaining void with a vodka of your choosing. Seal and place in cupboard for 30 days. You will want to shake, invert, mix this every few days to help the sugar dissolve. I try to leave as little headspace as possible in this step.
After 30 days, filter the contents of the jar with a jelly filter, cheesecloth, or dirty old sock if need be. Pour this liqueur into a cleaned bottle of your choice, cap it and forget about it for at least 3 months. Seriously, this needs time to age. 3 months is ok, 6 months is great, and anything beyond that just continues to let it mellow.
I've tried similar recipes with lemons, raspberries, strawberries, chokecherries, pineapple, peaches, and pears. My favorites are lemon, pear, peach, and chokecherry. You can tweak the recipe as you wish, if using something like chokecherries, you may wish to go up to 2 cups of sugar. You can also jack up the alcohol content a little by using 5 parts vodka and 1 part Everclear, but be careful with that one.