Openly-licensed question banks, activities, etc. that are comparable to what traditional textbook publishers offer aren't always available. One alternative is to ask students to draft questions of the text. Here is an example of how this was done in a psychology course. The questions can then be used or revised each year by the next round of students. The idea is that it's an effective pedagogical exercise for students.

Open Sources for Ancillary Materials

OER Commons: It has a little bit of everything. Use the Advanced Search feature to refine your search to a specific format type.
MERLOT: an open educational resource project from the California State University system that started in 1997
LibreText: an online textbook platform that also includes many ancillary materials
OpenStax CNX: a place to find ancillary materials for OpenStax textbooks
MyOpenMath: includes math problem sets

Commercial Products

Commercial products that utilize OER are also starting to become available. These products usually load OER into their platform and then "enhance" them with question banks and instructor activities. These are of particular interest to instructors who really need ancillary materials. Note that students may have to pay for these, either through a fee or with their data (i.e. they must create accounts). These are typically not OER and sometimes even lock open content into a platform that inhibits others from building upon it. Some common products include Lumen Learning Online Homework Manager, Intellus, and B&N LoudCloud. Abbey Elder created a Commercial Platforms spreadsheet that catalogs commercial tools that integrate OER into their platforms, which may be a good starting point. It was originally created in 2018 but it is being updated fairly regularly.