Lego buttercream cake. Make the Lego shapes out of fondant. Use fondant cutters for the rectangles and the small circles. Then add Lego people as if they are building the bricks.
Easy Lego cake. Use Reece's cups on top of rectangular cake, then frost! || I would do a light frosting layer then the peanut butter cups then frost again and perhaps stack two cups high for the peg height