[mjohnson]

See the things that you can't see elsewhere.

Perhaps Smithfield market, a hundreds of years old late night machine distributing to the endless commercial meat-slingers. Hanging meat, forklifts, (careful) public welcome and so on - then a meat-breakfast at a nearby market pub opening when the market closes. After many hundreds of years they're closing (gentrification) and moving to the burbs.

There might be a good/interesting produce and or fish-focused version of the same. Night busses are interesting at 0400 as long as you avoid drunk clubbers.

Grab a pre-sunrise breakfast at a 24/7 joint like Duck and Waffle. In the city near Liverpool St station, rez encouraged, 360-degree view of things like the gherkin and St Pauls, champs encouraged but likely 20GBP a glass.

See the Thames barrier and a nearby pedestrian tunnel under the river. Up top, grab a TfL ferry to get back to the other side of London.

Head upriver a bit to get cleaner water and hire a guided kayak trip.

Oxford is probably only 45 min from Paddington assuming the rail-things are working properly. Hop off one stop early and walk the last six or so miles along the Thames and into town.

Cocktails at the Booking Office at St Pancras station is a "must-do" for me.

The Silver Vaults are pretty cool, and in the city center. Free to enter, they do check your bags and you might want to bring sunglasses for all of the glitter.

If you're flying out of LHR I advise staying near the airport the night before. If so, and you're staying at one of the bigger/corporate places in Bath Rd, there's a bowling alley about even with the airport's east boundary. Given more time, Windsor/Eton is an easy bus ride or even a walk from the hotels immediately to the west by the M4.

(we're planning a trip for July due to a spouse's business stuff - this time kiddo and I may head off to Wales).