Walking is best way to look and feel city during your visit or holiday. Another, also very healthy and fun way of green city sightseeing is riding a bike. Most popular cities, like Paris or Barcelona has very developed public bike network. Paris has most developed public bike network (Velib) and it is great success. Of course they have problems with vandals and steeling bike but they show right way of green tourism. Barcelona has similar model, also extremely successful but main difference is that their bike network can be used only by residence of Barcelona not tourist. For tourist there are tourist agencies with same service.
So what you need for green city sightseeing on bike. You should be healthy ready for some adventure. You should have credit card as assurance instrument. Most important is that you have travel route planned. You can use lot of travel attractions on this site to build route based on your preferences. Also check stations where you can return bike. If you like, you can combine riding a bike with public transportation (like metro).
And what is green city sightseeing in future. Well defiantly public bike networks in cities should be expanded. Paris is thinking about public network of Small electrical automobiles.
Write about your experience using those networks and write about green travel and green vacation.