OK, this is a difficult one as London is large place depending on which areas are included, and there are a lot of travel brochures and guides - most of the more obvious things are taken. So looking at more central areas, some of the favourite things for my family while out for the day include:
1) Having Ice cream at Fortnum and Mason's Ice Cream Palour on the second floor. A Knickerbocker Glory is about £15 UKP, so a little expensive, but a nice place to take a hour out.
2) Having lunch or dinner in the restaurant at the top of the Tate modern. If you get a table near the window (floor to ceiling glass), the view is great.
3) Alternatively the restaurant in the Oxo tower for dinner, again the view is good, and you can often see famous faces (at least in the UK) in the bar area.
4) The view from the Thames from a speedboat/powerboat. The starting point is under the London Eye. The amphibious "Duck" trip is also worthwhile.
5) If you're taking children to the British Musuem, go down into the Ford Children's Centre (it's underneath the main atrium). During the week it caters for school trips, but at weekends it usually has several family activities running. At any time ask to borrow one of the children's activity packs. The one for Egyptian room contains soft-play human organs to help explain the process of making a mummy.
6) The Wellcome Trust building on Euston Road is also worth a visit. The permanent galleries are excellent, particularly Dr. Wellcome's medical equipment collection, but they also have regular new exhibitions - the current one is an exhibit of the wax medical training "bodies" used to explain anatomy - free entry.
7) Take advantage of the underground network and explore to the end of the lines. The London Underground is the oldest and I believe longest underground network in the world.
8) The small parks or "squares" in many parts of central London are always a good place to spend lunchtime, particularly if you've been in areas such as Oxford Street. Russell Square is ok, but busy, Tavistock or Gordon Square can be more pleasant.
9) Sections of east London are currently being 'regenerated' as part of the Olympic preparations - there are tours and walks around the new construction and stadia.
10) Ok, the final one definitely will not be in a travel brochure. The revolving restaurant at the top of the BT tower is planned to re-open in 2011. It's been closed since the early 1980s.