Just a quick code snippet which adds an extension method for drawing Rectangles to SpriteBatch:

public static class SpriteBatchHelper
{
    static Texture2D pixel;

    private static void LoadPixel(GraphicsDevice graphicsDevice)
    {
        if(pixel == null)
        {
            pixel = new Texture2D(graphicsDevice, 1, 1);
            pixel.SetData<Color>(new Color[] { Color.White });
        }
    }

    public static void DrawRectangle(this SpriteBatch spriteBatch, Rectangle rectangle, Color color)
    {
        LoadPixel(spriteBatch.GraphicsDevice);
        spriteBatch.Draw(pixel, rectangle, color);
    }
}