You could create end caps by creating custom arrowheads.
[Adobe video tutorial link removed. They changed it to some "pop art" tutorial.]
You can follow the Adobe Help Pages, here's what they say:
To define custom arrowheads, open the Arrowheads.ai file, which is
located under ShowPackageContent\Required\Resources<locale>\ (for
Mac) and \Support Files\Required\Resources<locale>\ (for Windows).
Follow the instructions in the file to create custom arrowheads.
Place the updated Arrowheads.ai file at: \Plug-ins
and avoid replacing the existing Arrowheads.ai file.
Note, you must relaunch Illustrator after editing the Arrowheads.ai file (or replacing it).
Then apply the arrowhead to the strokes via the Stroke Panel.
You could also create brushes with end caps (pattern brush) but arrowheads offer the advantage of scaling with the stroke properly and brushes can be hit or miss when it comes to scaling ends without middle pieces.
Art Brushes, with the ability to
Stretch Between Guides can be a quick and easy way to create some end items....
I use what is needed and sometimes the brushes just don't work as expected, especially for non-perpendicular or curved paths. The arrowheads seem to always work as expected.