I have a constant stream of people visiting me in London, so I've heard lots of feedback. Much of it depends on how adventurous and well-traveled your guests are. If they are more comfortable with structure, then I'd suggest the Original London Tour, but only the Yellow line with the live guide, it's much better. It departs on Whitcomb St. near Piccadilly Circus by the McDonald's. It's only about a couple hours round trip and gives a great overview and visual perspective of London, and I always learn something new about London each time, even as a resident. Then maybe something will perk their interest and they can be spontaneous about what to visit.
http://www.theoriginaltour.com/tour-information/tour-routes.htmIf they are more into walking, I highly recommend London Walks Tours. You just pick and topic and show up. It's also a couple hours. I've gone on most of them over my years here and really enjoy them.
http://www.walks.com/If they are more independent, I recommend the No. 6 bus. Catch it in Covent Garden on the Strand and hot off at sights along the way, Trafalgar Square, Piccadilly Circus, shopping on Regent and Oxford St, Marble Arch (Speaker's Corner and Hyde Park), Edgware Road and mid-eastern eateries and shisha, Church Street Market, Maida Vale (Clifton Road high street) and Little Venice. They can get off at Warwick Avenue and walk around the canals there. This is a good combination of seeing a large variety of both touristy and local things. End up at one of the pubs in Maida Vale like the Prince Alfred, Warrington, or Warwick Castle.
If they have time for just one museum, the British Museum seems to appeal to everyone and no one is ever let down. Plus, there are some good pubs around there that will impress like the Princess Louise and Museum Pub.
If they are more edgy: maybe Hackney (like Broadway Market in Dalston), Deptford, Smithfields, Camden Town. If they are more mild: Sloan Square, Marylebone, Notting Hill.
The Thames Clipper is a good way to see the sights along the Thames easily and fairly inexpensively.
Something non touristy in London? That's tough to be honest because there are tourists everywhere. Maybe get on the Tube, get out of Zone 1, pick a stop and explore the high street. If you want to see a real cross-section of London, the Wembley IKEA at Brent Cross will do the trick.
Kidding aside, maybe some of the areas around Hampstead would fill the bill. They're appealing and not as many tourists...usually.
Have fun. I love visitors. It always allows me to rekindle my love for London by seeing it through their eyes, even if I've become a bit jaded over time.