Last time I looked you generally end up waiting a good hour or so for the London Eye. Cable Cars I've done as well.
Personally, I reckon both the London Eye and the cable car are much overrated.
I've sometimes encouraged visitors to go on bits of the railway that are up above ground level, to see some sights - eg some of the DLR, bits of the underground out west. And of course there's the top deck of a bus [those that haven't been culled, that is...].
If you're into both historic transport and (attempted) upgrades, there's the SSL bits of the underground (District, Hammersmith & City, Circle, Met) - on some of those routes you can see bits of almost-untouched-for-150-years infrastructure, combined with the daily service disruptions on account of the problems with the switch to a new signalling system. Something for everyone!
For non-transport things - well, central London is stuffed with free culture ... BM, BL, galleries, ... And there are usually a few lively demonstrations going on somewhere around Whitehall.
