On our recent trip to London we were staying close to the London City Airport and so took the opportunity to take the Emirates Air line cable car from Royal Victoria across to the Greenwich Peninsula (and back!).
You can ride the Emirates Air line from near the O2 or from the other side of the Thames (DLR Royal Victoria). It costs just £4.50 each way, which is really good value. This is much more than just a mode of transport for getting across the river. It is great way to see this end of the Thames with views of the O2, Canary Wharf and the city.
It is much better value and a better experience than the London Eye, which will set you back £25 a ticket.
At the Greenwich terminal there is also an Emirates Aviation Experience attraction for which you can get a combined Air Line and Experience ticket. In the aviation experience you can experience being a piece of baggage or learn about jet engines. Or to fly an airbus simulator will cost a few more pounds.
Also worth noting that the O2 now houses a shopping mall and many restaurants in its outer skirts. So even more reason to visit this area of the city which in many places is basically a building site.