Day out with DS (13) and DM (80) within an hour of Bristol

[MrsMoastyToasty]

Looking for ideas for a day out tomorrow with DS (13) and my DM (80) up to an hour's travel from Bristol or Bath. We live locally so have done most of what both cities have to offer and don't fancy going down to the coast.
DS has Asperger's and loves anything to do with trains or water, but hates heights and crowds.
DM is generally fit for her age, but cannot walk too far without resting.
I'm thinking north of the M4 corridor.
Any ideas?

Slimbridge?

We've been to Slimbridge several times.

Cardiff?

The railway museum in Swindon?

Longleat Safari Park
Cotswold Water Park
Gloucester Cathedral is good for film and TV fans - it was used for scenes in Harry Potter, Mary Queen of Scots, Dr Who, Sherlock and Wolf Hall. The historic Gloucester Docks are just a short walk away and have also been used in a few films. canalrivertrust.org.uk/places-to-visit/gloucester-docks
National Waterways Museum canalrivertrust.org.uk/places-to-visit/national-waterways-museum
Westonbirt Arboretum

https://www.hauserwirth.com/locations/10068-hauser-wirth-somerset
This is a lovely day out.

Clearwell Caverns and Puzzle Wood (also Dr Who location) in Forest of Dean? With a nice pub lunch? Tintern Abbey if you have time.

Either Swindon Railway museum or the one at Didcot. The one at Didcot has steam trains on some days. Both easy to reach by train.

Also the Swindon and Cricklade railway runs steam trains most weekends.

Concorde museum

The day out was yesterday.

What did you do in the end, OP?

We went to Chepstow and had a look around the castle before going on a picnic.
Thanks for the suggestions. Some I hadn't thought of, so will look at those another time. A few ww have done before Steam, National Waterways and Concorde (which is only about a mile from DMs house so a regular visitor).

Have you done the Oakham Treasures?