The truck tour is just a drive up the track they made for the main zip wire and back again stopping to admire the view and be told some history. Can't imagine anyone paying 25+ quid for that...but no doubt some do or else Dolphin Capital / Taylor wouldn't offer it.
Public footpaths skirt the North and East boundaries and allow access to the Quarry Hospital area (fenced off for stabilisation work last time I walked around there) and the compressor house beloved by 'urban explorers'.
Active quarry site - contact them and ask about open days and group (which is unlikely for two of you!) visits as they have done both.
Felin Fawr slate works is a buisness park so is easy enough to walk around. Some of the buildings are little altered on the outside but there's little to see really. Best part is the waterwheels but there is no official access to those.
The other bits you can't get permission to visit so access falls outside BP. Easy enough to find on 'urbex' websites/goontube.