I second (or third) the idea about sitting upstairs on a bus to get the best view of the Christmas lights. I have some fantastic pictures. I didn't even mind rush hour – yes, the bus is very slow, but it gives you a chance to see all the window displays, watch the hordes of people on the street (from above the fray), etc. Very festive! And you can hop out at, say, Selfridges if you feel like it.
Edit: We did do the London Eye when our boys were small (like 8 & 10), and they loved it. But as people have said, there are great views to be had elsewhere unless someone in your party is set on the Eye. We haven't done it yet, but we've heard the view of St Paul's (& London) from a nearby building, One New Change, is fantastic – and free.