Slice a thin layer off the bottom of the watermelon so it sits flat and doesn't roll on the plate. Using a marker or toothpick, sketch a simple jack-o-lantern face with a wide, jagged mouth. Carefully cut along your lines with a sharp paring knife, keeping the opening large enough to fit a spoon.
Once the face is cut, use your serving spoon to scoop out all the watermelon flesh in chunks. Try to keep the shell walls fairly sturdy so the carved face holds its shape through the party. Set the scooped watermelon chunks aside in your mixing bowl as you work.
While the watermelon shell drains a little, chop the pineapple and honeydew into bite-sized pieces of a similar size. Peel and separate the clementines into segments, and gently rinse the berries under cool water. Pat everything dry before it goes anywhere near the bowl.
Toss the watermelon, pineapple, honeydew, and clementines together in the large bowl, then fold in the blueberries gently. Add the blackberries last so they stay whole and don't streak the other fruit purple. Spoon the mixed fruit into the watermelon's mouth so it looks like it's spilling out onto the plate.
Scatter extra fruit around the base of the watermelon on the serving plate for that "overflowing" effect. Tuck a few fresh mint leaves into the gaps for a pop of green against the orange and red tones. Serve immediately or keep chilled until your guests arrive.