He is the best narrator we have. Alec McCowen enters in tweed plus-fours, lights a pipe and announces: "All children grow up, except one." Three hours of enchantment follow, reminding anyone who may have scoured too many critical-theory books that shows are at their most theatrical when most childlike. It's only dressing up and make-believe: on the first night of Peter Pan there was a child who wouldn't grow up inside every one of the audience.

As the Storyteller, McCowen doesn't patronise, twinkle, or cosy up to us. Beady-eyed, he delivers JM Barrie's lines with crispness, attack and irony. You could listen to him do the whole book. As it is, John Caird's production has a more generous budget, and soon widens out to take us from Bloomsbury to Never Land, the Mermaids' Lagoon and Pirate Ship, before catching the next flight home. John Napier's designs rival McCowen's performance: the chimneys, roofs and oval windows of the Darling household glide in out of the gloom like a turn-of-the-century spaceship. When Wendy, John and Michael learn to fly, the bedroom walls revolve; they soar through the third-storey window and out over the stalls. A vast gauze scrim circles round with the sky and clouds projected on it - high up in the Olivier tower, stars are shining. Peter Pan has taken wing - and, as it does so, you could almost hear the new Trevor Nunn regime heave a collective sigh. It has its first hit on its hands.

Never Land slides into view as an idyllic theme park, with cacti, palm trees, blossom, wild grass, rocks, toadstools, sunflowers, and a totem-pole with a bowler hat. When we move to the echoey Lagoon, mermaids recline in rocky alcoves, while the children dive off a skull into billowing sheets. Two fins of a shark circle round the lagoon as if following its own Scaletrix track.

The jokes are fresh as ever: what other playwright could match Barrie in capturing the fierce logic of a child's mind? Never Land is "second to the right and straight on till morning"; the Piccaninnies are led by Great Big Little Panther; and Slightly discovered his name from the tag sown inside his trousers ("slightly soiled"). While Caird's production, which he co-directs with Fiona Laird, exploits the Olivier's modern resources, it never swamps the Edwardian whimsy.

Ian McKellen is excellent twice: as Mr Darling, the amiable buffer, "working himself to the bone" to amuse his children, and as Captain Hook, a dashingly cavalier figure with glistening tunic and Old Etonian tie. In a clever piece of casting, Jenny Agutter plays Mrs Darling,combining in our minds memories of a child actor with a picture of motherhood. Claudie Blakley is a buoyantly pink-cheeked Wendy, and Daniel Evans, the Welsh-accented Peter Pan, has a winning directness and vigour. Clive Rowe, daintily entering as the bespectacled pirate Smee, has lost none of his knack for stealing scenes.

It's a big show. As it enters its fourth hour, you may wonder whether Never Land hasn't metamorphosed into Forever and Ever Land. But it's never dull. And if 10.30pm is way past some people's bedtimes, as McCowen remarks, once the Darling children have seen McKellen devoured by the crocodile: "The lateness of the hour was almost the biggest thing of all."

Over at the Donmar, the brims of hats tilt low over the eyes, the knots of ties hang a regulation two inches beneath the unbuttoned collars, and pencil stubs find a natural home behind the ears. The air thickens with smoke, wisecracks and hard-boiled camaraderie. The guy from the American wants to deal a new deck. The guy from Journal of Commerce tosses the receiver into his palm, puts a leg on the desk and scribbles on the back of his cigarette packet. The guy from City Press, his shirt spilling over his trousers, burps as he waddles out of the toilet. Every detail, from the crumpled pinstripes to the wooden swivel chairs and roll-top desk, glows with romance. Nostalgia has you in its grip when you lust after the props and furniture. But then The Front Page - the Broadway comedy by Ben Hecht and Charles MacArthur - is one of the great love stories. Seeing this, you can't help but fall madly for journalism.

This is Chicago, 1928; the hacks wear stubble as a badge of professionalism, and the main concern is to get a state hanging moved forward to 5am so it can make the City edition. Aw, it's swell. Sam Mendes once again demonstrates, as he did in the map-room scene in Othello, that he can orchestrate a crack ensemble team, with enough painterly detail to make a room seethe and breathe with life. Mendes is a master of hubbub. The question is: can he do "funny"?

Because if he can, why does he commit a dumb stinko act like casting Griff Rhys Jones as Hildy Johnson, the ace reporter, about to quit the newspaper game, get married and work in advertising? The guy's from the wrong genre. Rhys Jones enters with hat, cane and spruce three-piece suit. The lapels on the waistcoat are by no means the fanciest touch. His wrists flutter, his chin juts forward and his eyelids go blink-blink-blink. Yup. Into the lovingly recreated Press Room of the Criminal Courts Building, Chicago, steps that high-pitched chap from Holsten Pils.

By casting Rhys Jones as Hildy, Mendes rips The Front Page down the middle. On one side we have the newshounds: superbly played, with lip- smacking, grainy authenticity by Martin Marquez, Keith Bartlett, Hilton McRae, Ian Gelder, Nicholas Gleaves and Mark Benton. On the other side we have comedy performances: many of them perfectly decent in their way; many of which wouldn't look out of place in an early-evening sitcom. They have that starched feel of characters that are based on characters you've seen somewhere else. In the middle, we have Alun Armstrong as Hildy's boss, with bulbous eyes and bushy eyebrows, giving a sulphorously funny study in single-mindedness ("To hell with the Chinese Earthquake ... I don't care if there's a million dead.") In this Front Page, the hacks are on to a helluva story. The rest is just colour.

The town of Paisley in west Scotland yields to few places in Great Britain (outside the Palace of Westminster) in its talent for internecine warfare and political corruption. So it was a smart move by Jonathan Kent to commission Paisley playwright John Byrne, author of The Slab Boys Trilogy and Tutti Frutti, to write a version of The Government Inspector. Gogol's satire is still set in 1830s provincial Russia only the cast - excepting the foppishly cherubic Tom Hollander as Khlestakov and Brian Murphy as his endearingly down-to-earth servant Osip - speak in the kind of vibrantly frank Scots accents that probably haunt Tony Blair in the early hours.

The Almeida cast fling themselves into Byrne's version as if leaping off the Tarpeian rock. The speed and verve of the performances account for a five-act play coming in under two and a half hours. As the Lord Provost, the awesomely splenetic Ian McDiarmid borrows his tempo from commentator Julian Wilson, round about the time the horses enter the final furlong. What's odd is that smugness and self-esteem, the bourgeois veneer that settles over small-town corruption, never gets a chance to assert itself. So it isn't very funny. It's as if Kent has thrown everything at the punchline and forgotten to give us the feed.

'Peter Pan': Olivier, SE1 (0171 928 2252), to Mar. 'Front Page': Donmar WC2 (0171 369 1732), to 28 Feb. 'Government Inspector': Almeida, N1 (0171 359 4404), to 31 Jan.