What to do at LHR?

The big question is WHEN? The Olympics crowds may impact times, as well as the current 'labor conflict/slowdown'. If you don't have difficulties related to that, you MIGHT see the following (IF your plane is one time): You are arriving at a very busy time of the day- you may have to circle or wait for a gate, or might be parked at a remote gate and bussed to the terminal. If you are lucky, you'll be deplaned and in immigration line by 9:00 am.
Depending on how many other flights have arrived at the same approximate time, you may have from a 15 min. to an hour wait. If things go reasonably well, you will be through before 10:00am. This will give you about 3 1/2 hours before you need to think about finding your next gate (assuming you don't need to check-in again and go through security- if you do, then you'll only have 2 to 2 1/2 hours). There's lots to do. Plenty of shops and restaurants. And lots of airline lounges. If you don't have access to one via miles or other sources, it might be worth the money to buy a day pass. You can rest, eat some snacks, etc.

It's probably not enough time to do much outside the airport, especially with your bags, so I'd suggest spending the time in Terminal 5, the newest one and full of shopping aimed at the oil-rich Arab customers.
Gucci, Prada, Vuitton, etc. plus humbler fare, including food and of course Starbucks. You can spend a few thousand very fast or just window-shop and save the money for your trip. Nice modern light-filled place. But be warned, they charge a lot for wi-fi which IMHO should be free. We killed a few hours there last year on our way home.

Jennifer, Depending on the crowds at the time, you may not get through immigration and to your departure terminal until about 9:30. Although I didn't have that much of a layover when I dealt with that same situation last year, my first priority was a fine hot meal. Nothing like a good English "fry up" to get the day started, so I ordered the Full English Breakfast. I was ready for a meal at that point, as the "Continental Breakfast" served shortly before landing wasn't much of a meal. You could either do some shopping after breakfast, or have a short nap (perhaps one can sleep and one can watch the luggage, then change places). Happy travels!

When I saw this question, my first thought was, Shop and then Eat. ; ) There are a lot of stores, so take time to explore. I would actually recommend that you look at the bookshops and find a book that is not marketed in the US and use that as vacation reading. Have a nice lunch. I think that the suggestion of purchasing a day pass to airline lounge is a good idea. The alcohol is free in the American Lounge. ; ) Pam

Here is a shopping guide to Heathrow. Just click on the various Terminals under "Shopping & Leisure" to see the shops that each offers. http://www.heathrow-airport-guide.co.uk/shops-t-1.html You could stay busy a long time in the shops and restaurants. Harrod's, Burberry, Clarks shoes, Timberland clothing, luggage stores, jewelry stores, and Boots just in Terminal One alone.

Normally, that would sound like a generous layover, but right now it may be just enough time. LHR has been experiencing excessive wait times for a number of reasons (you can search online for articles about this). My parents flew home from Paris via LHR (on to KC!) last weekend and said the immigration lines at LHR were unbelievable2-3 hour wait. Their flight from Paris was late, so they were super lucky that a staff member took them to an expedited lane. Even that line took over 20 min. They were glad they had checked their luggage through for the trip home. Security lines were also huge and the less stuff you had the easier it was to get through. They noted that lots of (carry on) bags were being unpacked for inspection and lots of patdowns were slowing things even more. So, I'd say, consider yourself pleasantly lucky if you get through with short lines, and enjoy your lounge time in the terminal. We like the Giraffe restaurant for reasonably priced meals.

I agree with the others, by the time you're in Terminal 5 I don't think you'll have that long waiting around. Maybe get some UK newspapers or magazines to read and make yourself comfortable.
Terminal 5 is better than the others, more space and light, so hopefully you won't want to climb the walls!

In T5, have a leisurely lunch at the Gordon Ramsay restaurant, Plane Food.

Thanks for the replies! We will be there on Aug. 8th, so I imagine we'll run into some Olympics traffic. If we only bring a carry-on, can we bypass immigration and transfer from 3 to 5 in the secure area? The flights are not connected. We have roundtrip air to/from LHR, but bought a separate flight on BA to CDG.

All international transfers can be done airside without immigration. There will be a security check to go through, which has a queue all of its own.

Make sure to check in online for BA so that you have your boarding pass.

Here is Heathrow Airport's website page about connecting, with all the details of just what you will have to do. You can put your details into the "step-by-step guide," then watch the video or look at the FAQ's: http://www.heathrowairport.com/heathrow-airport-guide/flight-connections

gordon Ramsey has a restaurant at terminal 5, at least a year ago called flight or some other dumb name in (my opion) ask !! sort of name dropper thing but food ,breakfast was fine not cheap but where in London is cheap take US price multiply by 2.