A great walk down an old abandoned rail way from Finsbury Park to Highgate. Most of the original platforms are sill remaining so its an interesting place to explore...
The Greenwich Observatory with its green laser beam can transfigure any night sky. If you decide to cross the Thames via the foot tunnel, make sure you look out for the green ray. You can be on one side of the meridian in a minute, and cross to the other side on the next, without even realising it.
By far the best bagels in London, they are open 24 hours and being just down the road from my flat a bit too local. It is run by a man called Mr Sammy, who doesn't muck about with portion size. It is well worth a visit especially for their smoked salmon and cream cheese speciality.