I have my three lenses that I carry around everywhere.
The Canon 50mm 1.8 II, I am on a budget and it is a great lens for portraits and walking around, it's light and amazing.
The Sigma 105mm F/2.8 Macro EX DG, sharper then the Canon 100mm L, lighter and only cost me $300 on Ebay.
The Canon 17-40mm F/4 L USM, Wide angle to regular, very sharp, great for landscapes, the lightest L lens.
With the three lenses above, I have every focal length I need and they are all light and pretty cheap.