Line the bottom of a springform pan with parchment paper or wax paper for easy removal of the cake when done.
For the green/rind layer: Allow the green ice cream/sorbet to soften slightly. Take small scoops and arrange in a ring around the outside of the pan. Use a spatula to smooth into a 'rind' about 1 inch wide by 2 inches tall. If ice cream gets too melty, pop pan in freezer for 10-15 minutes to firm up, then continue reshaping. When perfect, return to freezer and allow to harden completely.
For the white layer: Allow the white ice cream/sorbet to soften slightly. Repeat the process, but make the layer thinner, about 1/2 inch wide. If it gets too melty, freeze briefly and reshape. Once shaped, freeze until hardened.
For the pink watermelon middle: Allow the pink ice cream/sorbet to soften. Transfer to a medium bowl and mix in the chocolate chips. Spoon the mixture evenly into the middle of the prepared 'watermelon rind.' Top decoratively with the remaining 1 tablespoon chocolate chips. Return to freezer until fully re-frozen.
When ready to serve, release the springform pan, cut into watermelon 'wedges', and enjoy!