If I've got a few hours to kill in London I always go looks around Harrods. I recommend eating in the foodhall, it's expensive but great. They sell some really nice take out sandwiches which are reasonable and you can go find a park somewhere and have a little picnic. The whole of the store is pretty good and I have always found the toy department to be much better than Hamleys.

I follow this up with a tube ride over to Covent Garden (if you feel energetic try taking the stairs instead of the lift from the tube station!) watch what is going on there for a while, then take a walk over to Leicester Square and onto Piccadilly Circus for a burger in the window of the Burger King under all the signs at night.

I am not a fan of London, I have been lucky to have visited many times for both business and pleasure and the novelty has worn off. But I would say approach the city assuming you will come back some day and don't cram too much into your day. I think popping into the odd little pub you see is also a good idea, take your time enjoy the beer and the city don't do the normal tourist thing of trying to fit the whole city in, it's just not going to happen so you may as well enjoy fully what you can see.

Cheers

Cris.