Sadly, the environment remains a rather low priority in Beijing. The city suffers from incredible levels of pollution, has a staggering population of 17.4 million (though many people believe this to be much higher given the influx of unregistered migrant workers), and has a growing middle class eager to buy private cars and contribute to the CO2 emissions. As a visitor here, you will have to make a very conscious effort to travel green.
Taxis are plentiful and cheap, but this is the original city of bicycles! Beijing is a sprawling landscape, but it is flat as a pancake and larger boulevards -- Chang'an Jie, for example -- have excellent bicycle lanes. During rush hour, biking is often faster than taking a cab. Most hotels rent bicycles at affordable day rates. You'll also find entrepreneurial xiaomaibu, or convenience stalls/newspaper stands, renting bicycles. Several xiaomaibu beside popular subway stops (Jian Guo Men, exit A; Liangmaqiao, exit B, among others) rent reliable wheels for ¥2 per hour or ¥20 per day, plus a ¥200 deposit. Be sure to try the bike and test the lock first.
A great way to cut down on your own emissions is to fiddle with the temperature control in your hotel room. High-end chains often have the central-air units turned down to a chilly temperature (19°C/66°F?!) during the hot summer months. Ceiling fans are much greener options, or a more energy-efficient temperature between 21°C and 23°C (70°F-73°F).
Street bins with dual containers for recyclables and trash are starting to show up on Beijing's major streets. The underground recycling trade here is prolific, however, so there's no need to hang on to empty water bottles in an attempt to find the appropriate recycling bin -- just leave it on top of a trash can or (I know, it's hard) on the street and a bottle collector will pounce upon it soon enough.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.