Are there any special sights in London that are suited for the very different interest of a family consisting of 5 persons between 20 and 60 years old?
I don't think that museums are a good choice.
I think you'd be pleasantly surprised by the museums - they're VERY interesting and not at all dry, especially the new Darwin Center at the Natural History Museum.
People tend to be quite polarised about the London Eye - but the ability to go up high above central London in a glass ball is fun for all.
Richmond Park (if walking is ok) is a great trip just outside of London. Take the train to Richmond (20 min on the fast train) and walk up the hill - it's a beautiful view as you walk up. Then in the park, there are two herds of deer - Red and Fallow - about 600 in total. My goal is always to find the deer - it's a huge park and not always immediately obvious where they've moved to on any given day, but you will find them if you look. Stop for a break at the ice-cream vendor in the middle, just above the lake on the eastern side of the park.
And if you are into seeing animals, London Zoo has an amazing collection, and all types - be it mammals, reptiles, birds, fish or insects that you might want to see. World famous, highlights for me was seeing a live Okapi, and the giant anteaters!.
Borough Market is fantastic - it's been around for nearly 1000 years, just on Southbank. It's open from Thursday to Saturday, and the cheeses are fantastic! Also have a Monmouth coffee at the southern end of the market. Afterwards, a walk up to the Thames and along Southbank is a really nice walk in either direction.
Many of London tourist attractions are within walking distance of each other. The website London For Free outlines a variety of different walk that you may be interested in including:
To name but a few. You can also print out guided maps. The website also details other free activities to see & do in London & is well worth visiting.
For the very latest visit its Facebook page:
Take the Docklands Light Railway from the Bank. The trip is scenic past some very modern buildings built around the old docks of London. It is also cool with driver free trains for the technically interested.
Get off at Island Gardens and walk west along the water to Greenwich foot tunnel. An old tunnel under the Thames with elevators or stairs in both ends. Quite an experience.
When you emerge on the southern bank of the river - head for Greenwich Royal observatory. Beware of the museum (pretty interesting if you like a bit of history) but stroll in the gardens and get a glimpse of the Greenwich Meridian that defines the time of everyone.
Once done with sightseeing grab a pint at the cosy pub Plume of Feathers just north of the observatory gardens.