Long layover at Heathrow - what to do?

You could probably take a cab to Windsor and walk around and have a meal. I’d want to leave Windsor 4 hours before departure so you are well within the 3+ hr range for International flight departures. You’ll have to process out thru Immigration and then come back in thru security of course.

Are you checking bags?

Thanks for the suggestion! Yes our bags will be checked through; just carry on, So we will be quite mobile…

Use one of the various ways to get into an airline club and chill, your actual layover isn't 10 hours

I would definitely go into London. You can get there quickly via the Heathrow Express. Not inexpensive, but buying tickets ahead of time inline can save you some, seems to me a one-day round trip ticket is available for less. This will take you into central London, Paddington Station. From there you can take the underground, a taxi somewhere, or just walk around and explore. Check an online map and think about what you’d like to see. It will also be good for your body to move around between flights. Just allow plenty of time to return and go through those cursed lines at the airport. You can also take the underground from the airport, but it takes a lot longer. Have fun!

Take the Circle Line Underground train from Paddington to Westminster Station. Exit. You’ll be at Parliament Square. If you so desire walk over to Westminster Abbey and take a tour.

Afterwards follow Birdcage walk adjacent to St James Park up to the Palace. If the Royal Standard flies it means the King is in residence.

From there cross past the Queen Victoria statue (with the light…don’t jaywalk) into Green Park. Through Green Park to Piccadilly and then have afternoon tea at The Wolsley. If tea isn’t your thing walk from Green Park to Liberty to shop. If shopping isn’t in your wheelhouse then from St James Park try to locate the small and traditional Red Lion pub in the Crown Passage.

Or from Westminster follow the Thames River path till you get bored.