I’m not disputing that many ( non adopted , not traumatised , emotionally stable, securely attached and well behaved ) three year olds enjoy trips to museums or on the tube.
However , this child will only have been home for 5 months, you will still be getting to know him and his cues , likes and dislikes. On paper he may be three , but emotionally he will be much younger.
He will still be grieving the loss of his former carers and anxious and worried about when he will next be moved. He will probably be very stressed when tired or over stimulated ( like on a long and busy day trip to London ).
He won’t know the other adult/s present on that day because his grandfather lives abroad. Your father is strict and has high expectations of children’s behaviour.
You want to have a “nice lunch” - I assume this means a place that doesn’t serve chicken nuggets or other things the child is used to and won’t be set up for young children.
You don’t even know if your son will be settled enough to be left with your husband for the day ( I’m assuming that you OP will be the main carer).
You plan to give the child very little access to technology , even though he will be used to this at the FC so it will be comforting and familiar.
You say you want him to enjoy “ normal child’s play “ yet you want to take him to museums and art galleries.
I don’t want to be harsh, but that’s doesn’t sound very child centred to me. It sounds like it’s about the behavioural standards that you want the child to achieve and the type of interests that you want him to have, rather than you meeting his needs.
What if he only likes fish finger and spaghetti hoops ? What if he hates museums and only wants to run around in the park or watch TTTE or Peppa Pig on the iPad ? What is he’s scared of trains and won’t be left with dad, only mum ?
You are trying to fit this unknown child into your mould and that’s usually a recipe for disaster. You need to fit your lives and expectations around him and not the other way around. This isn’t like giving birth.
I’m sorry, I know you will not want to hear this. And I’m sure there are adopters out there who brought home a model child who did exactly what they wanted all the time from day one. It’s just I’ve never met or known of anyone like that .