Where there aren't crowds, an "out-and-back" routing can also offer various loops of less than 18 holes.
On Ballybunion Old you can play any number of holes from 2 to 18 finishing very near the clubhouse. The 14th, 16th, and 18th are close to home, and you can easily re-route yourself from the 1st, 2nd, 5th, 7th, 9th and even the 11th greens. It's a common practice every evening.
On many links, such as The Old Course at St. Andrews, your options are limited to much simpler loops. Machrihanish is similar in this way to TOC. You can play 2, 4, 8, 10, 12, and 15 hole loops. But you always finish on #18.