The Last Witch, Royal Lyceum, EdinburghLove Letters Straight from Your Heart, Dean's Room, McEwan Hall, EdinburghPower Plant, Royal Botanic Garden, Edinburgh

Rona Munro's true-life drama about a defiant single mother, and some moving experimentation, puts a bit of fire back into the festival

Last Sunday, I prophesied (after much cursing) that things could only get better in the Edinburgh International Festival's theatre programming. And as if by magic, an instant improvement.

The Last Witch by Rona Munro is a very strong new play – co-produced by the EIF and the Traverse Theatre Company – based on an intriguing, real-life case from 1727. Janet Horne was the last person to be burnt for sorcery in Scotland, and her legendary last words – quipping "That's a bonny warming" as she approached the pyre – have inspired Munro. The result is a portrait of a woman whose defiant spirit, coupled with poverty and irrepressible playfulness, causes her downfall.

Janet (an excellent Kathryn Howden) is a voluptuous, capricious single mother with an earthily poetic turn of phrase. Dirt poor in a rural backwater, she chastises her lame and disgruntled daughter for "moaning like a wet wind", while she herself merrily indulges in flights of fancy, pretending she can turn into a bee and blight her neighbours' cattle – so they bring her victuals.

But Janet is a fabulist amongst dangerously superstitious farmers, and a mocker of authority when the screwed-up witch-hunter, Andy Clark's Ross, comes calling.

Dominic Hill's production needs more fine-tuning. George Anton, depicting Jane's vengeful neighbour and jailer, is histrionic. However, Clark becomes startlingly passionate when he succumbs to Howden's buxom charms, and The Last Witch slides boldly into hallucinatory scenes.

Munro also plays clever games with our modern scepticism, letting us laugh at old, irrational beliefs before introducing subtle, devilish twists. Of course, it's hard to compete with The Crucible. But with a nod to Macbeth and Knives in Hens as well as Arthur Miller, The Last Witch will surely enter the Scottish canon of modern classics.

Meanwhile, the last week of the Fringe Festival has been boosted by a flurry of experimental shows, presented under the banner of the adventurous production team Fuel. These aren't uniformly brilliant, but Love Letters Straight from Your Heart – devised by the troupe Uninvited Guests – is astonishingly moving.

The set-up is a wedding reception where the bride and groom quirkily turn into rival DJs. They play pop songs pre-selected by the audience, who sit round a long table, sipping cava, while their individual dedications – to their loved ones – are read out (having been sent in beforehand by email).

It sounds like a droll gimmick. Yet the dedications are manifestly authentic and heartbreakingly sincere. At the performance I attended, there were testimonies of love for an adored girlfriend; for a disabled child; for a paternally abused and suicidal sister. I don't think I have ever wept so much in a show, just seeing others quietly trying not to cry when it came to their contribution.

Some might feel more exposed than they anticipated. Avoid the floodlit central seats. However, the sense of collective sympathy created (without any group-therapy chat) is profoundly comforting, and it's combined with surprisingly delightful light relief. Gaze into the eyes of the stranger opposite you for a full minute and enjoy a slow-dance at the end.

Finally, after being sorely disappointed three weeks back by the afternoon show Susurrus, in the Royal Botanic Garden, I returned to its steamy glasshouses late one night to catch Power Plant (created by Mark Anderson, Jony Easterby and others). This is a wonderful, witty promenade: a sort of magical/ mad scientists' son et lumière; a Martian art installation amidst the tropical foliage.

The palm house pulsates softly, with an eerie glow. Light bulbs whirr around in the trees, like fireflies, with a mystifying electromagnetic life of their own. A gramophone plays a record made of artificial grass, emitting a low gurgle. Organ pipes, scattered amidst the bushes, let out puffs of flame like an elfin oil field, and they create funky, tooting music at the same time. Floral evening dresses, in shreds, hang over a glimmering lily pond. Old lamps, amid fronds, flicker on and off as you pass, making galactic chirruping noises as if sending messages to another world. Absolutely bewitching.