Not sure what "cheapest and easiest" means in actual dollars. Most of my street shooting is done with a "user" Leica M2. They can be picked up for around $600 or so on ebay. If that is too much money the already mentioned suggestions make sense. If not you can start with Russian lenses with an M adapter which in most cases deliver excellent picture quality and go for around $50. If you like the Leica M and range finder experience you could always move up to C/V lenses or Leica glass. My walking around camera is an M2 with a Canon 35/2 (LTM) and an adapter.