We enjoyed Leiden so much that we took Grandma and older brother Ben back on Wednesday. Market days are Wednesday and Saturday but the weekend one is the biggest and also has an antiques market which is nice. Crowds are higher and it is fuller on Saturday but there are more stalls for sure. Just depends what you want.
We also found plenty of boat trips from small one family style to larger groups in covered boats. Most last 1 to 1.5 hours. The smaller boat drivers are open to bartering with taking children for free etc.