Sadly, Hitchin market is somewhat in decline and has nowhere near as many as 100 stalls. Less than half that. The transport section needs updating as the forthcoming changes mentioned at the end of the article have already happened (not terribly successfully, by all accounts!).
I don't think the article mentions the British Schools Museum with examples of Victorian and Edwardian classrooms. There are lots of hands-on activities for kids and it has special themed days and exhibitions.
As well as Oughtonhead Nature Reserve, Wymondley Woods to the south of the town is lovely for walks. On the edge of the wood, there is a fishing lake called Coots Corner which is open all year round - they've recently been putting up posters advertising a cafe open to all visitors, not just people fishing.
Might be worth adding that Stevenage Leisure Park is very close (10-15 minutes drive) and has lots of family-friendly activities: 360 Indoor Play area (bigger than both the Hitchin ones), Mr Mulligan's World of Golf (indoor golf suitable for all ages), Airspace indoor trampolining, Cineworld and lots of places to eat.
Further afield, there are several wildlife parks, all within an hour's drive: Paradise Wildlife Park in Broxbourne, Shepreth Wildlife Park near Cambridge, Woburn Safari Park and Whipsnade Safari Park.
Fifteen minutes up the A1 there is Shuttleworth with its collection of vintage cars and aeroplanes, plus a big outdoor playground. If you like aeroplanes, Hitchin is also 30 minutes drive from the Duxford air museum, part of the Imperial War Museum.