Primarily a commuter ferry service between Manhattan, Brooklyn, and Queens, East River Ferry is also great for sightseeing. For $4 ($6 on weekends, bike is $1 extra), you can take a boat ride with sweeping view of the downtown Manhattan skyline and Brooklyn.
The ferry is often not crowded, and the ride is fast - it only takes about 30 minutes to get from the first to last stop (Wall Street to 34th Street, or vice versa). On summer weekends (May 26th - Sept. 30th) it can also take you to Governors Island. www.eastriverferry.com