Day out in London with 5 year old - can you help me plan a nice day please?

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I have a precious alone day with DS1 (5) before I go back to work and hIs two younger siblings are at nursery. We have never had a day out alone before and I thought a day in London would be good. I can get there for 10/11 ish and would really need to leave again at 5 ish (cn be a bit flexible). Think I'll buy a one day travelcard.
So, my first thought was nat hist museum (dino's) and maybe the science museum but I'm a bit worried about wasting precious time queuing?
Should I be doing something else? Any ideas?
I want him to soak in the atmosphere of a big city and have a day to remember, preferably without spending much other that travel /cafe lunch.
Thanks in advance.

Apparently the Princess Diana memorial garden/playground is excellent (if the weather is ok of course). My SiL took her 4 and 6 year olds on an open top bus recently as well so they got to see Buck Palace Big Ben etc without too much walking. Have fun!

I would recommend a boat trip or bus ride to avoid small tired legs....

That's a great idea will look into a bus/ boat tour.

I would worry about quese at the science and history museums, you just walk straight in

I wouldnt* duh

Things my almost-5-year-old loves: Nat History Museum, Aquarium (but this is v. expensive), Hyde Park (Diana playground, Diana fountain, Lido (which is next to Diana Fountain)), Hamleys. Open-top bus tour (or just riding on a bus - some of the ordinary buses go through the centre of London and are included in the cost of your travelcard).

If going before 9 th September go on the Mayor of London's website and download one or two of the routes showing where all the different Wenlock and Mandeville statues are. They are in famous places and decorated as beefeaters, big Ben etc. great way to see some of the sites and soak up some Olympic spirit! We have been twice and seen thirty odd, there are 80 and my lot are desperate to go again. There were plenty of other people out looking at them and taking photos.

The Southbank always has things going on.

Go to st James park with a bag full of nuts and feed the squirrels.

It was the (cheap) highlight of one of our London trips for 5 yr old dd

Anything within walking distance to a park is lovely.

A bus ride around St james- the Mall- picadilly. Soak up the athmosphere around embankment/trafalgar the bus back up to regents/park & the zoo.

Natural history museum + hyde park works anyday for me.

The bridges and coty see from
A boat + Southbank walk also very nice on a sunny day

Southbank is great at the moment for kids - there is a rainbow sandpit! And there are easily accessible loos, and somewhere to shelter if it rains (RFH).

The Golden Hinde ship by London Bridge has pirate days and workshops - my DSs loved it there aged 4/5. Second suggestion for boat ride - catch the boat to Greenwich from Westminister pier, then go to the National Maritime Museum and have a quick bite before you catch the boat, tube or train back into town.

Actually there is quite often queues at the museums especially at the weekends but they move pretty fast . Both well worth going though.

Thanks for all these great suggestions. He is very into Pirates so that sounds perfect for him. I like the south bank suggestion too (want to see rainbow sandpit) am going Thursday so maybe nat hist is an option?
My London geography is non existent though- what can I squeeze in between say 1030-430 getting in at Victoria?
Thanks

akaemmafrost i have been to both museums more times than i can remember and hsve not once que'd up??? I must be very very lucky

We did que for 'the bubble show' at the science museum (dont, its crap!) though?

Took my kids to the Natural History museum last October during half term holiday. Que was horrible, over an hour.

Tower of London is Brilliant but tiring. Just select the bits you want to see and save the rest for another day.

Well I never queue either at NHM because we always go in the lesser known side entrance .