A fun way to see Oxford that is environmentally friendly is on a rickshaw with the company, OxonCarts which can be found at www.oxoncarts.com. They will also take you to the restaurant of your choice in a unique style, so long as it is in the city centre!
There are parts of Oxford that visitors seem to miss, such as the riverside and canal and the University parks. The open top buses can only follow the main roads, but on foot or on a pedal-powered bike you can be taken down the cobbled back streets although the riverside you should see on foot.