Design challenges are constraint-based problems with multiple correct solutions. They may range from very simple (devise a classful addressing scheme for a network consisting of 2 PCs and 2 routers), to intermediate (devise a VLSM addressing scheme for a school with various classroom and administrative subnet needs), to complex (doing large parts of semester case studies in Packet Tracer). Some instructors have students use Packet Tracer to verify the functionality of IP addressing schemes they have designed. Design challenges are probably best done as blank or partially-completed .pkt files: given the open-ended nature of many design problems it may be difficult to author a graded .pka activity because the current version of the activity wizard, despite the variable manager, still has limited provisions for determining equivalence between the multiple correct answers that often occur in design problems.
The physical mode of Packet Tracer (with its Inter-City, City, Office, and Wiring Closet views), ability to load background images, and a variety of other annotation features (such as "i" boxes for network and scenario descriptions, customizable device names, place note tool, translatable GUI) may also be relevant for contextualizing case-study type design problems. Some instructors have students create designs in Packet Tracer and defend them in classroom design reviews before allowing students to implement them on real equipment.