My third years have recently spent some time discovering which sets of information describe unique triangles - in other words, finding the four congruencies for triangles. They used the information given to construct triangles on card and then cut them out to check if the triangles were the same or different.
This little activity from the National Library of Virtual Manipulatives does the same thing, but virtually. You are given lengths and angles and you have to try to construct different triangles with that information. You can soon see why the triangles have to be congruent!