If you aren't going far and hard, and if it's not stinking hot, all you need is water. For more demanding rides or if you are sweating profusely, you need to replace electrolytes. I favor Powerade or Gatorade, but some people buy tablets/powder and add it to their water. If you keep your eye out for sales (lots of 2-for-1 deals at gas station minimarts this time of year), Powerade isn't too expensive. Can't say I love the taste or the colors, but it isn't so bad.

Most all of us have one or two water bottle cages on our seat tubes and or down tubes. A few gulps/sips every 10-15 minutes, at traffic lights or while riding and we're good.