Begin by taking the softened cream cheese and placing it in a medium-sized mixing bowl. Add the powdered sugar and vanilla extract. Using an electric mixer or a whisk, blend the ingredients until smooth and creamy. In a separate bowl, gently fold the whipped cream into the cream cheese mixture, creating a light and fluffy filling.
Choose a rectangular or square dish to assemble your lasagna. Start by laying down a single layer of graham cracker sheets as the base.
Spread about one-third of the cream cheese and whipped cream mixture evenly over the graham cracker layer. Smooth it out with a spatula.
Scatter a layer of sliced strawberries and cubed watermelon evenly over the cream filling.
Repeat the process by adding another layer of graham crackers, followed by more cream filling, then another layer of strawberries and watermelon. For the final layer, top with graham crackers and spread the remaining cream filling on top. Smooth it out carefully.
Garnish the top with fresh mint leaves. Cover the dish with plastic wrap and refrigerate for at least 4 hours or overnight to allow the flavors to meld and the graham crackers to soften.