We love going into London and Jess is now at an age where she likes to join us. She loves the excitement of the train and visiting new places. I've recently discovered that there are lots of urban city farms in London and with the exception of Hounslow Urban Farm they are all free to visit. They just ask for a small donation.
A list of all the urban farms can be found here.
Remember that if you travel with more than two adults on First Capital Connect you can get a group saver ticket (you pay for the price of two adults and then one or two more adults can go free. In addition, up to four children can travel for just £1 each when accompanying a groupsave party).
If it's a nice day you could combine an urban farm with a picnic and play in one of Londons many parks and a stroll around a market and you have a lovely outdoors day out for the price of a train ticket.
I quite like the look of Freightliners Farm. I've heard that on Saturday's they have a market selling homemade food, arts, crafts and clothes.
From Hackney City Farm at Haggerston Park you can walk through Haggerston Park (with woodland walk, walled garden, pond and wild flower meadow) and Broadway Market (apparently the hippest shopping street in Hackney) to London Fields Lido (with heated pool!). The most highly rated of all Londons outdoor pools. It looks as though Hackney City Farm hold a bi-monthly arts and crafts fair too. Check their website for details.