Tourist traps are very much subjective. What one person finds tacky, over hyped and not worth the money or effort, the next person might absolutely love and find it one of their favourite memories. These are my own personal tourist traps in London. Your mileage may vary.
Picadilly Circus is a traffic bottle neck, Leicester square is pedestrian square a short distance away. There are a few large attractions on Picadilly like the Hippodrome and Rock Circus both of which are way over priced. The movie theatres on Leicester square are horrendously priced and some of the restaurants that we saw were as well. There are quite a few kitchy souvenir shops and kiosks too but souvenirs are a dime a dozen in London and you'll find nicer stuff elsewhere, probably cheaper too. It *is* pretty neat to see Picadilly circus lit up at night with all that neon. Sort of like Times Square but smaller, but when you see it once, that's all you need to do. Leicester Square has the half price theatre ticket booth but i personally prefer to call the theatre or book tickets on the internet to be sure to get what i want. For souvenir shopping, walk back a few blocks into Soho or go up along Bayswater road or near the Queensway tube stop. Same stuff, cheaper. Covent Garden has a lot of places as well and it's not far away. It's far more interesting for people watching as well.
Changing of the Guards All the tourist books advise you to go see the pomp and circumstance of the changing of the guard at Buckingham Palace. It's crowded, it's a bunch of soldiers marching around with someone shouting at them telling them what to do and where to go. Personally i think it's a waste of time. If you really must go, go early and get a good spot to see it rather than having to hop up and see over people's heads. Alternatively, If you want to see something similar, go see the HorseGuards. They have a changing of the guard as well and it's not far from the Palace. Probably less crowded and there are horses. Off Whitehall.
Harrod's Yes, Harrod's! Oh it's a lovely department store and you can get pretty much anything you want there. But it's VERY expensive. The food halls are an attraction but Fortnum and Mason has just as good if not quite as extensive. The cafes are pricey (but, ok, the food is pretty good). The whole store is so big that it's difficult to find things with various different sets of escalators and lifts around corners and then walking through many departments to get to the one you want or to a cafe.
They do have some lesser priced items like their own brand tea and coffee and they have some reasonably priced Christmas decorations in the fall. The food halls are interesting to look through, the Egyptian escalator are is worth a look as well though it does also have the Diana/Dodi memorial shrine at the bottom. Alternatively: Fortnum and Mason's on Piccadilly is an equally nice (and expensive) department store with a good food hall and nice cafes and far less crowded. Fortnum's was founded in 1707, more than 150 years before Harrod's. F&M also supplies the Royal Household by Royal Warrant which Harrod's does not have anymore so if it's prestige you want, it's there too. You can also order goods online at Fortnum's just as you can Harrod's so you aren't losing out there either.
High priced attractions
There are a few attractions in London that are *very* highly priced. Some of the highest tickets are for the Tower of London, Madame Toussauds, St. Paul's, London Eye, London Dungeons, Hampton Court, Kensington Palace, all starting from 10 pounds and up. That really adds up, especially if you have a group or family! Most of the major musuems and galleries are now free for the regular exhibits, with a fee for special and temporary exhibitions. Some, like the Dungeon are out and out rip offs. The London Aquarium has higher prices during school holidays and summer, another rip off. Most do have cheaper prices for children, students and seniors.
Having said that, I still found the Tower worth the entrance fee as well as Hampton Court (but remember there is transportation costs involved getting there as well) because there is quite a lot to see there and it could take you all day if you wanted to see it all.
The London Pass is a good idea if you are going to cram in a lot of sightseeing in a day but that's the whole thing, for you to break even or come out ahead, you have to schedule several things each day and that's a lot of sightseeing with a lot of pressure to "see it all". A one day pass at present is £19 for an adult and if you buy it online ahead of your trip, it comes with a one day central zone travel card, more savings!
You can buy a combination ticket to both the Tower and Hampton Court which will save you a little money.