6 months is a bit young, but Eureka (the museum for children) in Halifax will be great in a couple of years' time. (My dds are now 4 and 6 and love it, but probably first went when they were 2 and 4 and there was plenty for them to do even then, although a certain amount is lost on them at that age obviously.)
Nearby is also Shibden Park. It's a park with the usual play area (but a very good one) plus a boating lake, little train ride, and in the summer merry-go-rounds and things. Take a picnic, though, cos the food in the little cafe is naff.
Again, when your ds is a bit older, Tropical World in Leeds is nice. It's in Roundhay Park.
Also second Bolton Abbey, Yorkshire Sculpture Park, Cannon Hall, etc.
Had a fab day out today at Chatsworth, though that's a bit further afield (probably an hour and a half south of Leeds).