To get to lake Ilopango (inside the caldera) follow the Pan American highway east to the outskirts of San Salvador, past the military airport there is a turn off to the town of Apulo, which is also a recreational area (balneario). You can rent small boats to take around the lake or visit the Islas Quemadas, which are domes that were extruded in 1879-80, or drive around the edge of the lake to see a quarry in a rhyolite dome.