you can get an all day travel card which is a pass to go anywhere within central London via train or bus... that might save you a fair bit. I don't know if they do a family one though. A sight seeing trip on a bus followed by a sightseeing trip on a boat in the Thames (river) is a pretty good way to see a lot of the sights. Tower Bridge, The Tower of London, Houses of Parliament 10 downing street etc are all close to one another and can be a fairly pleasant walk along the waterfront. Also Greenwich Park is a nice area to pop into. The millenium wheel is also pretty popular but again expensive. Take a look at some London tourism sites as they tend to offer deals and discounts.
A lot of the museums are free (you may know that already.) I know you said no museums but the science museum tends to be very popular with kids. The Natural History museum is also very popular and both museums are free.
summitus: What ever you do , make sure you take a ride on the London Underground at rush hour ... best fun I can think of in London !! :p , If that fails the come up to North Wales for a day ... I live there , and it truly is the real Middle Earth ! :)
I'm going camping in North Wales (mix of wild and site) at the end of August. Love the place. Really couldn't recommend it enough. Yes the Welsh can be unfriendly but it's such a beautiful landscape. In the event you actually do drive there, try and head to a place called Lake Vyrnwy... most beautiful place I have ever been to and there's a waterfall (Rhiwargor Falls) hidden at one end. Sadly I won't be going there this year as we're exploring around Mt Snowdon.