Imagine Romeo and Juliet with the lovers played by Adolf Hitler and Eva Braun, tack on a happy ending, and you have a fair approximation of what happens in L'Incoronazione di Poppea - that dumbfounding product of the warped Renaissance mind, in which Monteverdi celebrates the triumph of eros over virtue. Like most scores of its age and fragmentary condition, it raises questions of text. But more than most, it raises a question of tone. At face value, Poppea is wilfully amoral, with comic touches. But to what extent should we read irony into its motives? Is it semi-serious libertarianism? Or a vaudeville to please the masses at a time when opera hadn't long been public property?

David Alden's new Welsh National production in Cardiff favours vaudeville: a Christmas-pantomime Poppea with applied jokes in unlikely places. And if the prospect of jokes from this scion of the school of searing modernism sounds unlikely - well, they're extremely funny; it's a good show; and the first night was a rare opportunity for Alden to take his curtain-call to cheers rather than boos. He looked surprised.

The standard apparatus of an Alden show is all there: loo walls, brutal lighting, bright red sofa, and a lot of cigarettes and whisky glasses. But the visuals are more securely stylish than in past productions, with amazing (if a touch camp) costumes, and sets that know when they need to respond to the sophistication of the score. The final scene - a setting for the jewel of the piece, the duet "Pur ti miro" - is a fine example: cleverly ambivalent between austerity and overstatement, with an embarras of chandeliers, transforming Nero's palace into Harvey Nichols lighting department. Another joke. But no worse than the joke of history, which is that "Pur ti miro" isn't actually by Monteverdi. It's the work of other hands: most likely his assistant.

The Cardiff cast is wonderful, with glorious drag acts from Neil Jenkins (great wigs) and Michael Chance (great legs). Sally Burgess (great singing) combines the comedy, nobility and pathos of Ottavia with flawless poise. Catrin Wyn Davies purrs the title role like a Renaissance sex kitten. And though there are arguments for alotting the originally castrato role of Nero to a female (if only to indulge the Straussian interplay of two high voices in that last duet), Paul Nilon makes a fine, light tenor job of it, incisively delineated with a darker, driven passion. Constantly on heat, he lives in his pyjamas. And they're quite becoming. Harvey Nichols vincit omnia.

The musical preparation has been superbly taken care of by Rinaldo Alessandrini, who uses a near-full edition of the score (ie long) and a comparatively big band in the pit. But nothing drags. The instrumental playing is transparent, and the buoyant, brisk attack sustains from start to finish. A complete joy, this Poppea tours to London next year; and the way things are operatically in London these days, it will be a breath of fresh air in the sick-bay.

Meanwhile, the city's lyric life somehow staggers on, and managed to offer three world premieres in the past week: each an exhumation of a score written some time ago but never staged. In order of entertainment value, they start with Stephen Sondheim's Saturday Night, which would have marked the composer's stage debut in the mid-1950s but someone died and the show was pulled. A later attempt at staging was cancelled by Sondheim himself. And it remained in a bottom drawer until the production that opened at the Bridewell Theatre on Wednesday.

I can see why Sondheim held back. It's a slight piece - a sort of Friends in period frocks - about a group of 1920s Brooklyn housemates looking for love. Much of the music is standard Fifties Broadway, with no distinctive Sondheim number until a love duet, "So many people", near the end of Act I. But the shape of Sondheim's melody - the scooping-upward leaps and angular descents - is embryonically apparent. So is the shape of his storytelling, with fantasy commentaries intercut into the action. And so is the tone: detached, ironic, puncturing passion with sour harmonies and sawing (as opposed to soaring) lines. The central characters edge awkwardly toward true love through self-delusion and pretence: it could be Wagner on the cheap. And there are more Wagnerian comparisons in the length of the piece (three hours) and its pace (slow). But that said, Saturday Night comes with show-stoppable lyrics that pour out of the mouths of characters who would never have such verbal virtuosity in real life. There's one vintage Sondheim song, "What more do I need?" And although this Bridewell premiere is modest, on a postage-stamp-sized stage that can't accommodate the fantasy effects and with cast-members not long out of theatre school, it captures the off-Broadway, workshop circumstances which the show seems to anticipate. It feels right. And beyond mere fascination, it's a joy to see and hear, with cute performances all round, especially from Tracie Bennett, Sam Newman and (cutest of all) James Millard.

The Donizetti premiere was more procedural than substantive, in that it involved not so much the unearthing of a piece as the unearthing of a revision of a piece. Otto Mesi in Due Ore was staged in Naples in 1827 and then adapted for Paris and London, by which time it had acquired a new name, Elisabetta. But Elisabetta never reached the stage, and the score was lost until the 1980s, when it surfaced at Covent Garden. After much rejoicing that the Garden's cupboards do occasionally hold something beyond skeletons, it was sent to Oxford University for touching up. And the result was given, in concert, on Tuesday at the RFH, with Carlo Rizzi conducting Royal Opera forces.

At this point, alas, the interest of Elisabetta fades. It's a conventional score that does no more than any theatre music of its time would do. With no love-interest beyond that of a daughter for her father (she walks from Siberia to Moscow to clear his good name - hence the original title), its emotional thrust is limited. And its smattering of humour (delivered by a buffo baritone in pale imitation of Rossini's Figaro) is thin and pointless. But the score is undeniably attractive, and was eloquently sung here by a fine cast: the Hungarian soprano Andrea Rost in golden- flute voice for the title role; British bass Alastair Miles on handsome form; and the young Peruvian tenore di grazia Juan Diego Florez, whose lean, importunate bel canto singing I reported earlier this year from the Rossini Festival at Pesaro. A real star on the rise.

There was nothing starry about Havergal Brian's The Cenci, which had its long-delayed premiere at the QEH last weekend. Written in 1952 by a maverick whose vast output (32 symphonies, five operas) wins him entry to the cult of the heroically forgotten, it amounts to two hours of relentless bluster: orchestrally competent but turgid in its handling of the text, which Brian was ill-advised to take direct from Shelley. With lacklustre conducting from James Kelleher, whose feel for balance, clarity and line was less than tender, the message of the evening was an old one: that obscure works generally deserve it.

'Saturday Night': Bridewell, EC4 (0171 936 3456), to 24 Jan.