Cold showers
wake you up thoroughly
increase circulation because blood flow must increase
stimulate and boost your immune system by increasing white blood cell production - people who routinely take cold showers have been shown to fall significantly less often ill with infectious diseases and infections
reduce muscle aches and soreness
Not only do I take cold showers in addition to luke-warm ones, I also spend hours and hours in the ocean every week, wearing either a tri-suit or a wetsuit. I'm in 52°F-60°F water, depending on the time of year. Can't even remember getting a cold, the flu, or anything.