I have always found the above task, suggested in the National Strategies exemplification examples (click on the above image for the pdf) an excellent one when looking at types of triangles and their properties. Students are required to find the number of unique triangles on a 3×3 grid. The task can be extended by asking students how many unique triangles there are on a 4×4 grid. A good discussion can be had on being systematic in approach.
The next time I teach it I will certainly use a Geoboard.
For GeoBoard resources and suggested activities continue reading here.