This thread seems to be mostly about coming to London to leave London.
there are some smaller museums -
whitewebbs in enfield - quite limited opening days / times, if we could be sure of a reasonable number of people then could always ask if they would do a group visit on a 'non opening' day
walthamstow pump house
brunel museum at rotherhithe
lt museum depot at acton has a few days a year when it's open - there's nothing listed yet for that late in the year, but they may not have nailed down this year's events yet.
the railway museum at north woolwich closed some years ago. the station building's still there, but not open to the public or anything.
but not sure any of these would really be a day's outing, but we could try and plan a day that involves a few things.
there's a few sites of interest / closed stations / ex railway lines, but nothing all that spectacular.
there's
heritage routemasters (now operated commercially and priced to match rather than part of the TFL network) on route T15 between charing cross and tower hill - at least some of them are heavily refurbished / re-engined so personally, meh
the only 'heritage' ish rolling stock in service anywhere is the 1972 trains on the bakerloo line and 1973 trains on the piccadilly line (the latter have got replacements on order) and again, personally, meh (i can remember them being the newest trains on the network...)
doing some sort of river boat thing's a possible - although not sure i fancy doing a speedboat.
the woolwich ferry's a bit niche, but it's very unreliable and prone to technical troubles and /or industrial disputes - it does have phases of not operating weekends.
ETA -
there's also
croydon airport, although opening days are a bit limited and don't match. again, if there's enough of us, they might do a private visit.
the thames / river police have a small
museum in wapping, normally open to pre-booked groups only
the
museum of london (docklands) has plenty on the river and docks.