Would members in other locations like to provide similar lists - perhaps "within about an hour" if you're based in another less dense location?
What I came up with for .. you guessed it ...
Melksham. Where the drive is likely to be between 45 minutes and an hour, flagged.
* With rides:Jungle Railway / Longleat Safari Railway
Coate Water (longer drive)
Swindon and Cricklade (longer drive)
East Somerset Railway (longer drive)
Avon Valley Railway (longer drive)
Brokerswood Miniature Railway (Gone?)
* Without rides:Two Tunnels footpath
Calne to Chippenham Path
Steam (longer drive)
Crofton Beam Engine (longer drive)