"Down Time" — Eruvin 28b
ר' זירא כי הוי חלש הוי אזיל ויתיב אפתחא דרבי יהודה בר רבי אמי אמר כי נפקי ועיילי רבנן איקום מקמייהו
In the course of its deliberations here, the Gemara tells us that when R' Zeira felt weak, he would sit outside the door of R' Yehuda, so that he could stand up in honor of the rabbis that entered and exited the house.
The Kav V'Yashar (Chap. 45) learns from this Gemara a general principle, that a person should strive to always occupy himself with mitzvos. He states that he received a tradition from "Anshei Ma'aseh" ("Men of Deeds") that when they found themselves idle — i.e., not involved in performing any specific mitzvah — they would finger their tzitzis and look at them. He explains that from a Kabbalistic perspective as well, looking at tzitzis is an activity of great weight which helps in the process of restoring the Divine Presence to its proper place in the world.
The Kav v'Yashar reveals a little of the Kabbalistic significance of looking at tzitzis, with a fascinating gematriya: A person looks at the tzitzis with two eyes. An eye, in Hebrew, is Ayin — the numerical equivalent of 130, so two eyes are the numerical equivalent of 260. The word tzizis itself (which is spelled ציצת in the Torah) is the numerical equivalent of 590. Together, the two numbers equal 850, the numerical equivalent of the word techeles. Hence, posits the Kav v'Yashar, a person who focuses his eyes on his tzitzis fulfills the purpose of techeles even in our day and age.