Double decker bus

A 3 year old will have no knowledge of a tourist bus she won’t understand the running speech or anything about architecture or history Just take her on an ordinary double decker and point things out to her
She ll love it

I took my 2 year old on a bus into town and back while grandma watched his brand new baby brother between feeds. He got to sit upstairs at the front.
It was the highlight of the year for him.

You could as Zonne suggests get the train from MK to Harrow and Wealdstone and get on a double decker London bus there.

I agree with Elegran your GD will be none the wiser where you go, she'll have climbed upstairs, hopefully be able to sit at the front and will love seeing shops and traffic from a different height.

She is 3 and not 13. A local double-decker will be fine. If you have none locally find out the nearest town which has them. You do not need to spend much money if any as you will have a free pass and she will also not need a ticket.

Could you get a cheap day return from MK, and go on a London bus?

Aren't there any ordinary service buses near you? Does it have to be a tourist bus? Your granddaughter won't care if it just goes from the library to the supermarket so long as she can climb the stairs.

We had visitors from the States who were thrilled to ride a British bus. The icing on the cake was a snowstorm that threatened to stop all traffic and leave us stranded twenty miles from home. We were worried sick about how to get back, but they were thrilled to be in a real double decker in real snow!

Pack a picnic and go on a 'mystery tour'. Doesn't need to be far - Willen lake maybe or somewhere along the Grand Union canal? Pack some bread for the ducklings too!