Here are few ideas. Explosive eruptions fragment the lava to make pyroclastic material (blocks, bombs, cinder, ash). This material tends to pile up close to the vent (some of it gets carried down wind). The slope of the flank of the volcano is related to the size of the volcanic material. Larger pieces have steeper slopes. Smaller pieces have gentler slopes. You might visit a local stone quarry and buy gravel and sand of different sizes. Ask you students to make piles of the different size materials and measure the angles of the slopes. Do they observe any correlation between particle size and slope angle? Do they think this would also be true for volcanoes? Repeat the experiment with any volcanic materials you can obtain (cinders or ash).