We love this place, and have done for years, but it's surprising how many people still don't know about it.
If you go round the back of St. Paul's Cathedral and cross over the road, you will see a glass shopping centre called One New Change. There is a lift in the centre of it which takes you up to the open-air roof terrace, where you can step out and enjoy a wonderful view of London. And the best thing about it is this: it's totally free!
Make sure you wander across the whole rooftop, because you'll miss some of the best sights otherwise. The best one is obviously St. Paul's Cathedral, which is just over the road. You get a really fantastic view of the upper levels from there, and can have a good look at the dome. You can also see the London Eye and The Shard. And if you take the time to look (because they're not easy!) then you can also see a distant view of Parliament and Westminster Cathedral.
Have a read of Craig's London blog before you go, because he's written a review of exactly what you can see. Feel free to ask him some questions as well, or you can talk about One New Change on the forum.
The roof terrace also has a cafe selling breakfast, lunch, dinner, plus a bar for when evening comes. Maybe you could time your visit with a meal?
If you enjoy this view from One New Change, then there are a couple of other great views nearby. The best one is probably from the Sky Garden (which is also free -- check out Craig's review of the Sky Garden, because it really is worth visiting), and you might like The Monument as well (assuming that you don't mind the climb). Craig has reviewed The Monument as well. The obvious observation deck is at the top of The Shard, but that one is quite pricey. But here's his review if you fancy it.
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