Thanks everyone, this is all brilliant info! I've gone quiet because I'm making copious notes, adding things to the spreadsheet of potential places to go, and checking prices, times etc.
Here's the new list so far of places to consider:
Pennywell Farm,
Kingsand and Cawsand,
Plymouth Lido,
Lappa Valley,
Carnglaze caverns,
House of Marbles,
Dartmoor Zoo,
Plymouth Aquarium (even if it's small it's 99% likely to be a hit because they've never been to one before).
Lost Gardens of Heligan
Buckfastleigh steam train and butterfly and otter sanctuary
Buckfast abbey
Tamar trails
This probably sounds silly but it didn't occur to me that there would be a functional rail service because we don't live anywhere near one and all the little branch lines around here got closed down in the 70s or 80s so they're mostly just scenic footpaths now. I'll look into trains.
I think I wanted to go all over the place because I've always wanted to visit the West Country and have never made the logistics work, so it feels a bit like a once in a lifetime trip. I need to slow down a bit and remember I can book next year for a different part.
We've been considering NT membership for next year (DC1 only just turned 5 so the children have been getting in for free so far and DH works very long hours so usually I only have to pay for my ticket) so will look at what NT properties are in the area.
I suppose now the risk is overplanning, but I always think having a massive pool of options is perfect to be more flexible if plans change (or the weather does).