Legally Blonde, The Musical, Savoy, London<br/>Trilogy, BAC, London <br/>Greta Garbo Came to Donegal, Tricycle, London

Elle is a Californian airhead, your stereotypical dumb blonde. The Chihuahua bobbing in her pink handbag, quite possibly, has a higher IQ.

Indeed, our ditzy rom-com heroine is only interested in fashion accessories and a marriage proposal from her swanky boyfriend, Warner, in Legally Blonde – the latest movie-turned-West End musical to declare itself an "unprecedented" hit while still in previews.

But whaddya know! Elle is gonna become a Harvard Law School star. Legally Blonde is like a ridiculous combo of Educating Rita and Erin Brockovich. Played by Duncan James (from the boy band Blue), Warner heads off into the Ivy League's fast lane, and ditches Sheridan Smith's Elle as a silly bit of fluff. However, she determines to pursue him, by gaining a place on the same course.

So, this is showbiz's radical antidote to dumbing down, right? Yeah, sure. Deciding she has to become serious, stop partying and attain top marks, Smith's Elle spends at least 60 seconds at a desk, head down, while chorus-line studs prance around topless, trying to distract her.

OK, maybe her cerebral workout wasn't quite sufficient. She ends up, surreally, getting into Harvard by treating the interview panel (all male, of course) to a shimmying routine, dressed as a spangly majorette. Later, in class, she makes friends with a clever, cute guy (Alex Gaumond) who persuades her to thumb through a text book, instead of glossy mags.

Next thing you know, she's a pinstriped trainee on her professor's select legal team, helping him win a high-profile murder case. Sheer genius, it ain't. She works out that a male witness must be lying about a hetero affair because, when she deliberately bends over to flaunt her butt in his face, he doesn't respond. Therefore, he must be gay. Hey-hey!

Legally Blonde is tongue-in-cheek, blithely post-PC, evidently just a bit of fun. However, before we are all rendered completely brain dead, I feel duty bound to mumble, under my breath, that I found it surreptitiously, or perhaps inadvertently, pernicious. Droves of adolescent girls are lapping up this show, some of them reportedly returning night after night. Yet their role model, Elle – though put through college – is never allowed to be properly, categorically brainy and drop all the bimbo business.

That said, Smith manages to totter around in her stilettos and act the caricatured dolly-bird with an ironic twinkle in her eye and delightful comic timing – outshining Reese Witherspoon in the movie. Gaumond and Jill Halfpenny, as Elle's hunk-craving hairdresser, are both likeable too. He can sing and she can't half dance.

Nonetheless, without Smith, Jerry Mitchell's production would lose almost all its charm. Most of the company just holler the pop-rock numbers for all they're worth – which isn't much.

Meanwhile, over in Battersea's BAC, old-school feminism appears to be back in full swing. Transferring from the Edinburgh Fringe – and going on to the Barbican – Trilogy is an award-winning experimental show. Performed by a handful of women in their twenties, plus one young man, it centres on film footage of a rowdy debate at New York Town Hall in 1971. We see out-takes, projected above the stage, of Germaine Greer grandstanding, Jill Johnston burbling some hippy prose-poem about every woman being a lesbian, and Norman Mailer being a noxiously snappy chairman. Meanwhile, on stage, Nic Green and her co-performers launch into dance moves, jiggling on the spot and making what might be obeisant bows.

Personally, I'd like to have watched the whole film (which will be screened at the Barbican) and participated in a proper discussion about it. Green and co's half-baked choreography lacks lucidity, and their bitty script only throws up a few thought-provoking questions, such as "Do we ever really celebrate difference, or is it only certain differences?"

Still, the great naked dance, in which a bevy of female volunteers of every shape and age flood on to the stage – leaping, bouncing and whooping – is wonderfully brave and joyous.

In Frank McGuinness's new drama, Greta Garbo Came to Donegal, everyone is wondering if the fiercely private Hollywood beauty will purchase the big house in this Irish backwater where she's visiting the English painter, Sir Matthew Dover. However, all is not serene here. The unmarried housekeeper, Michelle Fairley's Paulie, is trying to keep a cool head, but her boozy brother, the chauffeur, is at loggerheads with his wife, and picking fights with Sir Matthew's butch gay lover, Harry. Caroline Lagerfelt's Garbo (pictured left), an imperious sphinx, blows hot and cold, taking a flirtatious bisexual shine to Harry and Paulie. Meanwhile, this being 1967, renewed political battles are simmering in nearby Derry.

Nicolas Kent's production is set against a burning blue sky, with picnics on tinder-dry grass. Yet the dramatic temperature is slow to rise, and McGuinness's scenario can feel like Chekhov and Friel reheated. Still, Fairley is absorbingly quiet, repressing past griefs, and Lisa Diveney is outstanding as her fuming teenage niece, prevented by her parents from taking up a university scholarship and escaping them all.

'Legally Blonde' (0844 871 7687) to 23 May; 'Trilogy' at the Barbican (0845 120 7550) 22 & 23 Jan; 'Greta Garbo Came to Donegal' (020-7328 1000) to 20 Feb