Is there a simple way to add symbols (circles, triangles etc) to the end of a line in QGIS?
If you need it only for display:
Layer Properties | Style
(2) Click on green + button two times. Change each
Symbol layer type to
Marker line and check
on first vertex only and
on last vertex only.
(3) Default markers would be circles. To change them to triangles, scroll this window a little down to icon list and select your preferred one.