Look back over your geometry lesson notes and determine which portions of the curriculum you want to apply for the assignment. Some examples include perimeter, volume, area, certain building shapes, three-dimensional shape creation, surface area requirements, coordinate use, point plotting and angles used for certain structures. List out each requirement and how it should fit into the overall city plan. For younger geometry students, you may want to include any necessary formulas on the sheet as well.
Use blueprinting as both an application and a checkpoint for the assignment. Obtain some blueprint paper for the kids and encourage them to draw out their cities using compasses, carpenter's squares, carpenter's Ts, rulers and yardsticks. Encourage them to label everything and check off the requirements from their assignment list.
The next assignment checkpoint would be requiring the students to mark off the building spaces on their building board. Provide the students with a Styrofoam board for the base of their city. Their job is to take their blueprint and zone off the various areas to scale. This is especially useful for teaching students about congruent shapes.
The final step is getting the students to test out their designs by building the city. Give the students Popsicle sticks, glue and paint for building the city. The challenge is to see if their theoretical designs are actually sound enough to stand. You may want to open up your classroom for after-school building if the students need additional time.
As an added challenge for older children, you could have a contest to see whose designs stand under the most weight.