Day trip to London with DD aged 3 - any suggestions please?

[longbay]

Firstly i'm 25wks pregnant and don't plan on taking a pushchair. Maybe that's a bad idea?? Anyway, I'd like to take her somewhere fun. Was thinking maybe the Natural History museum or London Zoo. Is it worth taking her round Hamleys?
Anyway, please share your ideas with me.
Also any travel tips e.g once i arrive at London Marylebone; is it easier to take a bus or taxi instead of the underground?
I'm going this thursday.
You probably think I'm mad for going so close to the jubilee but it's actually half way to my in-laws on the south coast who we're staying with for the weekend. We either break up the journey doing this or go in the car and sit on the M25 for 5 hrs on Friday instead!!

Hamleys is hell but the natural history museum and zoo should be ok on a weekday - also science museum and museum of childhood. What about taking a boat trip down the Thames, london eye or fountains at Somerset House?

If you have good weather, two great parks are Corams Fields (Russell square tube station) and the "Peter pan" playground in Kensington gardens.
I always felt that the zoo was overpriced and not that great (but I haven't been for a few years so it may have been updated.)

If you want to do a museum, the natural history and science museums are always good, the children I have looked after have also really enjoyed the transport museum in Covent garden and the museum of childhood in bethnal green.

Another popular trip with preschoolers that I have done a few times is to get the Thames clipper boat from outside the Tate Britain, it's a twenty minute ride to the Tate modern and you will see the London eye, big Ben etc on the way. From the Tate modern you can walk down to Tower bridge on the Thames path. There is no traffic and there are usually a few street entertainers around. We always went on the Golden Hinde ship, near London bridge as well.

If you are planning on using public transport I wouldn't bother with a buggy.

Thanks for some great tips