There is so much going on along the South Bank this summer that you feel that if you went inside the National, or the Tate, or the Festival Hall, you'd be missing out on something.

A huge grey ark has just appeared in front of the National as part of the Watch This Space festival, which in turn is part of a National Theatre Inside Out summer programme (in partnership with American Express), which in further turn is part of the London 2012 festival. One good turn deserves another, I suppose, but this pile-up of collaboration does at least seem to be letting the artists get on with it.

And Sue Hill and Bill Mitchell of Wildworks are indeed doing just that inside the ark, creating an inter-active installation called Ark-ive, in which any passing person is invited to share animal stories and photographs, investigate their own animal tendencies and presumably claim a place on board before the flood comes.

I popped in yesterday afternoon and talked about living with dog phobia, my dislike of cats, my hatred and fear of moths and my enthusiasm for blackbirds, giraffes and dolphins. My colleague Susannah Clapp of The Observer was asked to define herself with three qualities - she immediately said she was speedy and talkative; and they said she was glamorous - and she was then digitally "animalised" and came out as a snow leopard, though the photographic image made her look like a werewolf. 

The ark outside the National Theatre
It's all the most tremendous fun, and the interior is packed with ceramic and fluffy animals, statistics and definitions, cuddly toys and raw meat effigies, finger puppets and bestial tattoos, hanging menageries and caged beasts, with a sound deck of storms and cries.

The vessel - like the NT's temporary Propstore bar round the corner (which serves a very good pulled pork and coleslaw sandwich, by the way) - has been made from recycled scenery, found materials and aluminium scaffolding and will eventually be re-constructed in Cornwall, where Mitchell and Hill and their colleagues will carry on towards a major outdoor theatrical piece in due course.

Before going on board, I found another improvised pop-up venue on the first floor terrace of the National: here, National Theatre Learning (the educational wing of the NT) was presenting Bryony Lavery's new play Cesario, a delightful tea-time dramatic meditation on kinship and twinship in Shakespeare, featuring Shakespeare's own twins, Judith and Hamnet, presaging the watery separation of Viola and Sebastian in Twelfth Night.

The company of young actors were drawn from schools and youth centres, all fully costumed in Elizabethan fig and running around a clever Stratford-upon-Avon design (by director Anthony Banks) of the Tudor walls and flower gardens at New Place. There were two big company set-pieces dividing the disputatious scenes between Shakespeare's children, a couple of builders' apprentices called Wattle and Daub, and the twin children of an apothecary, the girl twin done up as Gloriana.
 
The play was only 35 minutes long, but there was a lot packed in about sibling rivalry, separation and death, and the performance of little Lola Turner as Judith was quite terrifyingly good. Black drapes flapped about in the wind while members of the public and NT staff went about their business outside, the sun shone and a late summer babble of drowsy pleasure-making and pleasure-taking rose from the riverside below.

After a couple of hours, I dragged myself away as a folk band struck up in front of the ark and people lounging in deckchairs suddenly leapt to their feet and started dancing. The festival atmosphere will be reinforced tomorrow and Saturday when 250 members of the National Youth Theatre will join Ark-ive to bring the flood crashing into the green open air patch known as Theatre Square.

The statue of Laurence Olivier that is supposed to point out the National Theatre has been rendered sadly redundant, crowded out by people, noise, sponsors' logos, pop-up bars and now a massive dry-docked boat teeming with animal-lovers and chaps in black T-shirts wielding hammers and coiling ropes. Heave-ho, me hearties.