A measuring or rafter square is a great addition to any tool box. You can use it to mark a board for square or angled cuts. It fits securely into a tool belt and can be used to check the trueness of an angle or as a cutting guide for a circular saw. It's also handy when you're working with large-dimensional lumber or when you're laying out rafters.
The problem: you don’t have a mortar box or wheel-barrow to mix up those small batches of concrete. Instead, use the ring method on a slab or sheet of plywood. Put down coarse aggregate on the bottom, followed by layers of sand and cement. Reserve the center for a mixing well. Then add water to the center and pull the ingredients into it with a garden hoe.