The Science museum is in South Kensington, (nearest tube South Ken on the Picadilly line). It is free to enter and you can easily spend a whole afternoon here. I went in August during school holidays (without kids) and it was busy but bearable.
There is so much to see and do here , for both adults and children.
The Space section is one of the main sections and is very interesting however having visited the Science museum on a school trip when I was 8 I can safely say that some of the displays have not moved , changed or been refurbished in the last 23 years! Some of the space rockets really do look tacky - they are only models anyway but aged 8 I thought they were amazing whereas now since they really don't look like they have been kept in very good condition it really looks like some of them are made from empty toilet rolls and a load of tinfoil! That said kids will love it and there is lots of information and different displays on life in space. I realise that a museum will not update its displays very often but some of the space section is looking a bit dated thats all.

The flight gallery contains jet engines, other engines and models of planes.

Although the museum is free to get into there are lots of extras that you can pay for - when we went in August there was a Wallace and Grommit exhibition which was another £7 or £8 each to get in I think - it looked good but we didn't bother with it. These special exhibitions change quite often. There is also an IMAX 3D cinema with different films showing throughout the day and I think that is about £8 each too.

Other exhibitions are free and there was an F1 exhibition showing how F1 technology had assisted other areas of technology including medical technology which was really interested. There was a video from a team of surgeons at a hospital who had studied and visited a pit crew from an F1 team to learn about team work and how to work quickly without getting in each others way basically!

There are hands on displays throughout for kids and adults and also the Launch Pad section where it is really interactive and you can take part in mini experiments at different displays such as freezing water and looking at the patterns of the ice through a magnifying glass, and lots more. Unfortunately quite a lot of these were broken or being updated. This was a shame since as it was school holidays it was very busy with kids so a shame that many things were not working.

We easily spent 4 hours in the Science museum. My favourite part has always been the display of technology and everyday items over the years - it goes right up to the 1990's now and so contains items that I remember as being the latest technology which look so dated now - they even have a gameboy in the display and some early computers.

All in all a very interesting day out and cheap too since entry is free just need your tube or bus fare, plus there is an inside picnic area with benches to eat your own food so you don't have to spend in the cafe. The area wasn't particularly clean but it was ok and not too busy.