In addition to admission-free world-class museums, there are parks, canal walks, supermarkets and maybe even some royal-spotting to pass the time. The possibilities are endless.

Who cares if London's hotels and restaurants tend to be on the expensive side?

No city in the world has more free stuff to do. In addition to admission-free world-class museums, there are parks, canal walks, supermarkets (Portobello Road Market at Notting Hill Gate, Camden Market at Chalk Farm Road, Columbia Road Flower Market in East London) and maybe even some royal-spotting to pass the time. The possibilities are endless.

1. Borough Market
Since the 13th Century, Borough Market has been stuffed with food-lovers and is the perfect place for a memorable grab-and-go breakfast or lunch. Open Thursday to Saturday, it is one of the top attractions south of the river. Go on Saturday to catch the market at its bustling best.

2. British Film Institute's Mediatheque
Hidden under Waterloo Bridge, the institute features four cinemas (not free) and the fun Mediatheque, where you can peruse the DTV/film archives and watch for free.

3. British Museum
The British Museum is one of London's top attractions and is absolutely free. Hop in for 20 minutes, peek at the Rosetta Stone, and move on, saving Aztec mosaic masks or the head-smashed "Lindow Man" (a 1st-century unfortunate found in a peat bog in 1984) for another visit. Watch for worthwhile 20- and 50-minute eye-opener tours, also free.

4. Houses of Parliament
The Houses of Parliament, home of "Big Ben", is a neo-Gothic wonder from the mid 19th Century. And it is full of houses: namely the House of Commons and the House of Lords. Reserve ahead to watch the antics during Parliament sessions:

5. Museum of London
Off the radar to most visitors, yet one of the city's great attractions, this museum offers a walk through London's various incarnations - from Thames Valley geological history, to Anglo-Saxons and 21st-Century bankers. Plus there is a nice cafe in its garden.

6. National Gallery
The National Gallery is a serious art stop, with 2,000 Western European classics by Van Gogh, Renoir, Leonardo da Vinci and Michelangelo, and more than five million popping into the Trafalgar Square building each year. Skip the hordes by visiting on weekday mornings or Wednesday evenings (after 6 pm). Any time, it is all free.

7. National Maritime Museum
Greenwich's best attraction, this neoclassical building museum gets more interesting and fun the deeper you go in. The focus is on Britain's seafaring past, including the bullet that felled Horatio Nelson, a replica of Ernest Shackleton's life boat and plenty of kid-friendly interactive exhibits.

8. National Portrait Gallery
Before Google or Wikipedia, the English came to the National Portrait Gallery to put a face to the name of who's who in history. Here, a block north of the National Gallery on St Martin's Place, you will see paintings and sculptures, including Andy Warhol's take on the Queen.

9. Natural History Museum
The Natural History Museum is an outrageous collection of things nature, in a lovely Gothic Revival building from 1880. A diplodocus dinosaur skeleton watches the entrance. Farther in lies a T-Rex and the Darwin Centre, with 450,000 jars of pickled specimens. A wildlife garden is open April to September only.

10. Photographers' Gallery
This wonderful contemporary photo collection in the West End benefits from its new two-floor space - where the Photographers' Gallery has been since 2008. Plus there is a great café:

11. Science Museum
The highly informative and entertaining Science Museum fills seven floors with interactive exhibits. The Energy Hall highlights the first steam locomotives of the early 19th Century. Popular with kids are the third floor exhibits, including old gliders, hot-air balloons and flight simulators.

12. Serpentine Gallery
Designed like a 1930s-style tearoom in leafy Kensington Gardens, the Serpentine Gallery is a lovely spot to take in one of London's most important contemporary art collections, with works by Damien Hirst and Jeff Koons. Loads of natural light pour through huge windows. Each year a new "Summer Pavilion" (May to October) is opened nearby, to host open-air cinema and readings.

13. Sir John Soane's Museum
Son of a bricklayer, Soane's West End home is filled with the early 19th-century architect's personal effects and curiosities, making up one of London's most atmospheric and fascinating sights that few know exists. The house is largely as Soane left it upon his death in 1837, from Christopher Wren drawings, a lantern room and slaves' chains. Aim to go on the first Tuesday of the month, when the home is lit by candles.

14. St Paul's Church
Not to be confused with St Paul's Cathedral (a big attraction that comes with a ticket price), this church on the western flank of Covent Garden Piazza is known as the "actor's church". The first Punch and Judy show took place in 1662, and there are memorials for Charlie Chaplin and Vivien Leigh, the most famous faux British Confederate of all time.

15. Tate Britain
The older half of the Tate duo (the modern bits moved upriver in 2000) is no stodgy sister. Here, permanent works focus on British masterpieces from the 16th to late 20th Centuries. Look for one-hour thematic tours and 15-minute talks on painters and paintings, all part of the admission price: nothing.

16. Tate Modern
Speaking of which, this mod half of the Tate, hiply set in the Bankside Power Station on the Thames, is one of the city's most beloved attractions. Special exhibits cost £8 to £10, but you can spend much time enjoying its permanent (free) collection of 60,000 works, including Pollock, Warhol, Rodin and Matisse, wrapped in uniquely themed exhibits such as "Poetry and Dream" for the surreal. The upstairs cafe has wonderful Thames views.

17. Temple Church
Da Vinci Code fans make Temple Church - with origins dating to the 12th Century - a must-see in London, for its role in a key scene. It is a distinctive place, built by crusading monks, with a traffic-free oasis of green spaces amidst the buildings in the city.

18. Victoria and Albert Museum
Open since 1852, its 4.5 million objects - like the stunner entry chandelier by Dale Chihuly - make it, very simply, the world's best decorative arts museum. Block out plenty of time. The first floor focuses on Asian (Japanese swords, ancient Chinese ceramics) and some European art, including plaster casts Michelangelo used for his David. Then there is a photography collection of half a million images, picked up over its 160 years. The Ardabil Carpet in the Middle East-focused Jameel Gallery is one of the world's oldest, dating from Iran in the 1500s.

19. Wallace Collection
Arguably London's finest small gallery, and way off the normal map, the collection is an enthralling glimpse into 18th-century aristocratic life, set up in a sumptuous restored Italianate mansion stuffed with 17th- and 18th-century art works.

20. Whitechapel Gallery
Home to 10 galleries in an art nouveau building first opened in 1899, the Whitechapel Gallery varies its themed exhibits between established and emerging artists. Picasso's Guernica was first shown here in 1939. Watch for music, readings and films on Thursdays and some Fridays, or pop into the uberdesigned cafe for a break.

The article 'Twenty free attractions in London' was published in partnership with Lonely Planet.