Internal, Mercure Point Hotel, Edinburgh<br/>East 10th Street, Traverse, Edinburgh<br/>Sea Wall, Traverse, Edinburgh<br/>Susurrus, Royal Botanic Gardens, Edinburgh

Speed dating: what a great idea. I mean what a terrific concept for an experimental theatre company at the Edinburgh Fringe Festival. After all, everybody is, supposedly, game for a drunken fling here – and I'm not talking Scottish dancing.

Internal is a guaranteed hit, being an extraordinarily intimate liaison between adventurous theatre-goers and a handful of avant-garde thespians. Devised by the award-winning Belgian troupe Ontroerend Goed (in league with the Traverse Theatre), it's a brief encounter in a darkened chamber at a hotel, with just five punters per performance and a matching number of actors and actresses in search of a partner (though watch out for extras masquerading as members of the public too).

After being lined up in the gloaming and eyed flirtatiously, I found myself steered into a curtained booth by a serious-faced, bespectacled young man – with his arm already slung lightly around me. Pouring me a schnapps, he inquired, in a soft voice, what I thought of him. Was he my type? He told me, in return, his impression of me (which was subtly provocative), before asking where I would ideally like to take him and – erm –had I ever imagined murdering anyone ....

Such cut-to-the-chase confidences are certainly unsettling. They contrive to be at once winningly direct and fraught with tension because – if you've any sense – you never quite trust your actor-date. Dramatic twists may well be just around the corner as you emerge from your booth and hook up again with the others, sitting in a circle for a teasing – or uncomfortably treacherous – group analysis session. This is a rich, edgy and illuminating little adventure. But give these guys your home address at your peril as they whisper au revoir and say they'll "do something nice with it". Yikes.

At the Traverse itself, East 10th Street is the New York actor Edgar Oliver's autobiographically rambling – or is it madly hallucinatory? – account of living in a gothic horror of a boarding house for years on end. Oliver's wavering, singsong voice seems irritatingly affected at first, but he becomes more mesmerising given time: a wan, spotlit wraith. Conjuring up a netherworld that's somewhere between Dostoevsky and the Munsters, Oliver offers vivid glimpses of his insane and derelict fellow-lodgers. He describes how they regularly crept around on the stairwells, naked, clung to the walls like flies, poured pickled excrement under his door and were carted off by the authorities, trussed to stretchers never to be seen again. East 10th Street is appallingly funny, oddly loving and, in the end, hauntingly forlorn.

Sea Wall, written by Simon Stephens, is another monologue, and is electrifyingly superb. The thirtysomething Irish actor Andrew Scott stands in jeans and T-shirt – with the working lights up – as if just casually chatting away, for real. Directed by George Perrin as a touring Bush Theatre production, Scott's performance is so natural – scratching his nose, pulling at an eyelash – that it's unnerving. He starts off talking with humorous fondness, almost like an observational comedian, about his wife and their beloved little girl Lucy; about his ex-military father-in-law and their seaside holidays together; about scuba diving around wrecks and startling sheer cliffs in the seabed. He jokes about whether God exists and might, defying all expectations, resemble Gary Glitter. Then, pausing, he suddenly drops his head and surreally remarks on the gaping hole which, he says, we can see running through his stomach.

His affable chinwag, before this, has been oddly choppy, perhaps faintly neurotic. But now the lurch into something far bleaker – describing a decimating family tragedy, involving Lucy and his stepfather – is like an emotional rip tide. This is a stagger-ingly harrowing piece of writing and a superlative production. Not to be missed.

Susurrus is – by contrast – a sorely disappointing experimental promenade, with headphones and strained references to Benjamin Britten's A Midsummer Night's Dream. This is supposed to be a site-specific treat in Edinburgh's Royal Botanic Garden (presented under the aegis of the Assembly Rooms), created by the writer-director David Leddy. He has been lauded as one of Scottish theatre's rising stars, but the firmament must be woefully dim then.

Don't get me wrong, the Botanic Garden is beautiful. How better to spend a sun-drenched afternoon than ambling around its winding paths, over grassy lawns and under towering cedars? But how nettling to have Leddy's barely relevant, pre-recorded drama playing in your ear, detracting from these horticultural delights. Basically, this is a C-rate radio play (on an iPod) where a brother and sister reminisce gloomily about the demise of their opera-singer father who was caught sexually abusing boys.

Leddy is bravely polemical in having the man's son suggest that subsequent psychotherapy sessions have scarred him more than his father's highland-holidays paedophilia, which felt like love. However, the additional voices which keep chipping in – a gossipy ex-diva in faux-docudrama mode, a cranky pathologist who dissects songbirds – are merely irritating. Ditch the drama. Enjoy the flora.

'Internal' to 30 Aug, 'East 10th Street' to 16 Aug; 'Sea Wall' (all 0131-228 1404) to 16 Aug; 'Susurrus' (0131-623 3030) to 6 Sep