Arriving 8:45am on BA from USA....flight out to Paris is 5:35pm so effectively have 9 hours minus the time for bureacracy of clearing passport control and returning security. Luggage will be carryon only.

Suggestions on ways to spend the time other than sitting in airport all day. I do have access to Business Class lounge but even that gets old after a couple of hours.

Appreciate any ideas offered.

Not 9 hours -- more like 7. Being in Business means you will use the fast track immigration so unless your flight is late you could be through arrivals before 9:30. There is Left Luggage at LHR so you can check your carry on bag there and not have to lug it. Do allow some extra time on your return - sometimes there is a queue at Left Luggage.

Again -- w/ business you get fast track security so you really don't have to be back at LHR until about 4PM or a little before if you leave your carry on at the airport.

No matter if you take the tube, a car service, or the HEX + a taxi it will basically take 1 hour to get anywhere interesting in central London and 1 hour to get back so you have approx 5 hours to play with.

That is enough for MANY different options. What sorts of things interest you? If you want to have lunch in a specific/posh place you will want to pre-book.

Another option is to head over to Windsor -- about 15-20 minutes by taxi. You could tour the castle and have a nice pub lunch.

Had I planned your itinerary, considering such a long layover, I'd have not flown to Paris but used the opportunity to spend the whole day in London, leaving on the Eurostar from London's St. Pancras station, arriving in central Paris at the Gare du Nord (city center to city center). Dinner either prior to the train or on arrival in Paris. If you can change it, I would. https://www.eurostar.com/us-en

Depends on your interest. A couple years ago I had a similar layover. As suggested by janisj I dropped my carry on at the baggage drop. Then headed to the train They have an express that leaves Heathrow for Paddington station like every 1/2 hour and gets you into London in 15 min.

Once in Paddington I walked out of the station and crossed the street to the Hop on Hop off big red bus. You can book online if you wish. This gives you transportation around town, you can hop off at any point of interest, then head back to the station.

There is also a nice pub across the street from the hop on stop outside Paddington. Great for a pint and fish and chips before you head back to the airport.

Sorry but I can't really second either of sm's suggestions. The HEX (the Heathrow Express train) is certainly fast in to town but it only get you to Paddington and there isn't much of interest nearby. There ARE Hyde Park/Kens Gardens and Little Venice. But if you want to get into very central London you'd still have to take a taxi or the tube so that 15 minute train ride still means about an hour to get anywhere interesting.

And I personally would not consider the H-o-H-o bus. It is slow, expensive, and you often sit for long stretches stuck in traffic. Plus if it is cold or wet - they are miserable.

If you want to head in to London, pick the one or two sites you want to visit and go there from the airport - depending on which sites they are, fastest is most likely the tube from LHR

We took the bus just outside where we left our luggage to the lovely town of Windsor. Took a boat ride there.., a long walk and nice lunch at a pub. It's 15 minutes away or so if I recall and very relaxing

Windsor is a good option but more like an hour by bus (leaves directly from T-5 where you will land). There used to be more/slightly faster bus routes but a cab would be much faster (and much more expensive)