Start by placing the gelatin powder in a small bowl. Whisk in the cold water thoroughly to ensure the gelatin is evenly distributed and starts to bloom. Quickly add the boiling water to the bloomed gelatin and whisk until completely dissolved. Set the mixture aside and allow it to cool to room temperature before using. I recommend whisking vigorously at this stage to avoid any lumps in your jello.

Slice the top off the watermelon and set the cut piece aside, as you will use it as a lid later. Using an ice cream scoop or a large spoon, carefully scoop out the watermelon flesh to create a hollow shell, making sure to leave the rind intact and sturdy enough to hold the filling. Be careful not to pierce the rind.

Set the hollowed watermelon shell into a bowl for stability. Fill the shell with the mixed fruit, distributing grapes, diced strawberries, blueberries, and some of the watermelon flesh evenly inside the cavity. This creates a colorful, juicy filling that will be suspended in the gelatin.

Carefully pour the cooled gelatin mixture over the prepared fruit in the watermelon shell, filling it to the top. Gently tap the watermelon to settle the gelatin and remove any air bubbles. Place the reserved watermelon lid back on top, cover securely, and refrigerate for at least 5 hours or preferably overnight to set the jello completely. For extra flavor, I sometimes add a splash of lemon juice to the gelatin before pouring it in.

Once the gelatin is fully set, remove the watermelon from the refrigerator. Cut the watermelon vertically in half, then slice into your desired serving sizes. Serve the colorful jello-filled watermelon slices cold for a refreshing treat—perfect for summer parties!