This engineering activity can be used on its own or before or after learning more about conductivity. There is enough background information for the activity to stand alone, but it makes a great engineering activity in the middle of a unit on electricity.
While I like to have the chance to assess students working independently, it is important to have students engineer in a group. The questions at the end of the activity concern the engineering process not the conductivity of materials. If you need something focused on conductance, see my conductivity lab.
You may need to use materials you already have, but if you can work with 9-volt batteries and bulbs from a string of holiday lights, those work great together. The higher voltage will complete a circuit with more materials that are in fact good conductors. The small bulbs are inexpensive, even if you have to buy a string of lights and pull out the bulbs (that may actually be the cheapest way to get them, if they are not on sale after the season).
I have included a photo of a couple of my students' conductivity testers. There are also Word and PDF files, so you can edit the activity if you like. The fourth page is three questions. It does not show in the preview.
I have listed this as a 2-hour activity, but you could definitely extend this to several hours, especially if combined with an exploration of the conductance of various materials and modifying the testers to be able to test liquids.
See the Q&A for some more details about materials.