Write up lecture notes, and design your own assignments. This is how a lot of my upper division math classes were setup, and the arrangement worked quite well. Textbooks are usually full of fluff not relevant to the class. Lecture notes, being designed by the person teaching the class, only contained material directly related to what was being lectured about / tested on. Of course, creating comprehensive lecture notes is not an easy task, but it's well worthwhile - write up your lecture notes once, and you can reuse them every year, without your students paying $100 for a textbook they'll never look at again.