Place the cranberries in a food processor and pulse until mostly chopped. Transfer them to a bowl and add the granulated sugar. Stir lightly, cover, and refrigerate overnight to help mellow the tartness and dissolve the sugar.

Remove the bowl from the refrigerator. Mix in the drained pineapple, grapes (if using), and mini marshmallows until evenly distributed.

Gently fold in the whipped heavy cream until the mixture is light and fluffy. Chill for an additional 30 minutes before serving. Garnish with extra marshmallows.