I find coming up with new tutorial, assignment, and exam questions. Choosing which questions to use is a challenge. Here are some questions I have used over the years in a fourth year undergraduate Condensed Matter Physics course based on Ashcroft and Mermin. I welcome comments on these questions and links to resources for other questions.
One useful resource [which I should look at more often] is Solid State Physics: Problems and Solutions by Mihaly and Martin.