Sprinkle 1 packet of unflavored gelatin over 1/4 cup cold water. Stir in 1/2 cup boiling water, sugar, and milk. Pour into an ice cube tray lightly coated with cooking spray and freeze until set, about 45 minutes.
Meanwhile, sprinkle 2 packets unflavored gelatin into 1/2 cup cold lemonade. Let it sit 1 minute, then stir in 1 1/2 cups boiling lemonade, lemon juice, and honey. Let the mixture cool to room temperature, then divide among four 8-ounce glasses. Refrigerate until beginning to set, about 75 minutes. Add gelatin cubes and a straw to each glass and return to the refrigerator to set completely, about 30 minutes. Garnish with lemon slices.