In the morning, pay a visit to Camden Market if you haven't been before. The Market is expanding, and I suspect that soon it will have swallowed up the whole of North London. So now is probably a good time to visit before everything else goes.
A lot of the goods on sale here do appear to be designed for small 12 year olds, even though apparently for adults, but amongst the same old, same old you'll find interesting art (I bought a beautiful Antarctica photograph by Andrew Scriven there last Christmas. I also recommend the Braintree Hemp Shop there and they often have reduced priced items of clothing in a rail outside.
This place is a must-see if you're travelling with teens and pre-teens. In my experience young girls could happily liveout their lives in Camden Market.
For lunch, get yourself up to the Bull & Last. Their fish and chips are fit for a king. Or if you're in the mood for something a bit lighter, there's a Japanese restaurant called Bento in Camden. They do a lovely selection of Bento boxes for around £7 each.
After your lunch a the Bull & Last, go for a stroll up to Parliament Hill to see the famous view - it's not very far away at all from the pub. From there, wander on to Hampstead Heath - a huge park to get happily lost in. I have heard there's a flock of flamingos there, but I've never seen them myself. I have seen parrots, though (my own picture above.)
If your lunch choice was Bento, then walk up Parkway to The Regent's Park (as it's officially called) to feed the ducks or attend an event (there is a regular schedule of events such as Taste of London and the Frieze Art Fair; the photo above was part of the Frieze Sculpture Park in Summer 2011).