It is chucking it down in Regent's Park, cars are splashing pedestrians, expensive frocks and suits are getting drenched. And, once you are inside the UK's biggest commercial art fair, you can add to the experience by buying your own dirty black puddle.

Each one of Marlie Mul's resin and sand hyperreal puddles would cost you €4,000 (£3,400)from the Frieze stand belonging to Fluxia, a young gallery based in Milan.

If you are not interested in splashing out on a puddle, then there are a further 151 contemporary art galleries displaying works in the vast tent that has appeared in the London park for the past 10 years.

As thousands descend on the fair – some buying, an awful lot more wishing they had the money to buy – hundreds of other galleries and museums open shows and have parties all over the capital. It is an art equivalent of the Japanese cherry blossom.

This year's Frieze is as dizzying and as diverse as ever. You can see astonishingly expensive works by Jeff Koons ("We don't release prices to the press," a Gagosian gallerist sniffed); and you can see far, far more affordable works by the possible next generation – for example post-graduate Goldsmiths students Sam Keogh and Joseph Noonan-Ganley, on display at Kerlin Gallery's stand.

And you can see porn. There are content warning notices but it is very easy to stumble into a screening of Omer Fast's Everything That Rises Must Converge, a four-channel film exploring the everyday and often ordinary lives of four hardcore porn film performers. It is being seen for the first time and does not spare viewers the graphic detail "although one of the things we found very compelling is the banality of the day at work", said Euridice Arratia of Berlin gallery Arratia Beer. "They are very regular people, there is an unexpected normality."

Arratia conceded it is a difficult work to sell – you would need €65,000 – but she hopes people will come and sit down and watch it.

A rather less in your face film is on display at Kate MacGarry's gallery stall. Marcus Coates – one of the artists shortlisted to be next up on London's Fourth Plinth – has made a film about hospice patient Alex.

Although the view is only from his window, the story is an epic one in that Alex told Coates he had always wanted to go the Amazon but obviously could not now – so could the artist go, which he did.

"It is a very moving film," said MacGarry. "It is a very strong work which says a lot about Marcus's wider practice."

Also on display are National Geographic-style photographs Coates has made of birds and animals and then screwed up and let unravel. Each are in an edition of three and would cost you £4,500.

Should visitors need a break from the crowds and intensity of Frieze, a good place could be a curvy glass pavilion installed as the only exhibit for the Lisson Gallery. Called Groovy Spiral, visitors are encouraged to walk into it.

Lisson's Ossian Ward said the work was a "people-watching experience. You feel like you're in the calm in the middle of the storm and you're on show as well. It is a nice place to be."

It is a meditative work, on Wednesday still for sale for $600,000 (£375,000). Ward said: "You don't get many moments for reflection at an art fair generally, a lot of what you see gets immediately forgotten, so it's nice to have that one moment. Frieze has matured so galleries should mature with the fair and feel confident to do big and bold statements and not feel they just have to chase after the money."

Frieze gets criticised because it is a place for conspicuous wealth and there may well will be oligarchs and hedge funders walking the aisles idly wondering what best to spend their millions on.

Matthew Slotover, fair co-founder, believes some of the criticisms are unfair."Some of the prices can get very high and if you don't like that then this is probably not the place for you," he said. "For me it is a wonderful thing that private collectors and museums buy art and support living artists so they can carry on making their work.

"Also commercial galleries all over the world offer hundreds of free shows every month."

With 2,000 artists at the fair it is also "an amazing opportunity to see a lot of what is going on".

He said a lot more big collectors were present this year, helped by the successful debut last year of a parallel fair for historical art called Frieze Masters, 15 minutes walk away in the park's north-west corner.

Here 130 galleries are selling their wares. For €22,000 you might be tempted by a Polynesian toggle, or for $1.5m an insanely kitsch example of Victorian narrative art, John Anster Fitzgerald's painting of Shakespeare's Bottom surrounded by fairies.

Or even a beautiful Brueghel winter scene, called The Census at Bethlehem, which, remarkably, was unknown and unrecorded until it was unveiled this week. The painting has been in the same family since it was bought direct from the artist's studio in 1611 and for the past 60 years has been in east Africa. "When we took it off the wall a mummified gecko fell off the stretcher," said Johnny Van Haeften, the Old Master dealer selling it.

It is Van Haeften's first Frieze – "I saw all my clients here last year and thought I'd better come" – and he is encouraged. "People who collect modern art are now beginning to look back at Old Masters which are so much cheaper by comparison. This is a great masterpiece by Brueghel and it is £6m – what does that buy you in contemporary art?"