I don't recommend the chocolate story. Its not really a museum. You will need to book tickets for a tour, its unlikely you could walk right in and get tickets straight away.
We brought tickets in the morning and couldn't get in until 3.30, luckily there is lots to do in York and we had lunch at Nando's.
You are herded into a big dark room, where the history of rowntrees is talked about, and given a chocolate drop, then into a room for a video about the history of chocolate, and given a thimble full of cocoa water.
the interactive part at the end where children can make a lollipop and there is a demonstration often gets backed up and a lot of waiting around. there are very small displays of old rowntrees/cravens/terrys packaging and some colouring in for the dc. We went in October half term and it was very busy, so I imagine this half term will be too.
The café downstairs is appalling, don't bother. The shop has some good gift items but very expensive really. I had high hopes for it as York has such a strong history of chocolate and sweets and we have strong family links with rowntrees as in generations of workers, also in terrys and cravens too.
Castle museum on the other hand is fab. we love it. Lots to see, the dc love the street and the more modern bit with the 60's street/shops.
The walls are a good walk and interesting in dry weather, miserable if wet. Shambles are good for a quick look, and a fab Italian there (bari).
Railway museum obviously, is the York wheel still there? not been for a few years to that museum.
betties is a must too, get there early for lunch with minimal queuing they have always been fab with the dc, good children's menu, the hot food there is delicious. I didn't think it was too badly priced either. approx. £80 for 5 of us.
we're there again in the summer, we have relatives living there, not far from the actual rowntrees factory. I lived there myself or a few years.