The Midrash (Shemos Rabbah 47:7) tells the following mashal: A king said to someone he loved, "Measure as many gold coins as you want and keep them for yourself." The Midrash says that this person didn't eat or sleep. We are certain about that because he wanted to take as many coins as he could. Similarly, the Midrash says that during the forty days that Moshe Rabbeinu was on Har Sinai receiving the Torah, he didn't eat, drink, or sleep.
Chazal (Succah 53.) tells us, "When we were in the Beis HaMikdash, celebrating the Simchas Beis HaShoeivah, our eyes didn't get tired." The Sfas Emes (5634) explains that they didn't want to sleep. The joy and the kedushah were available, and they didn't want to lose it by sleeping. Like the mashal of the Midrash (stated above), when a person can take a lot of good, he doesn't think about eating and sleeping.
The Beis Yisrael zt'l said that the same applies to Yom Tov. It should be so precious to us that we don't want to miss a moment.
Wasting time is the worst of all. This was taught by Reb Yechezkel of Kuzmir zt'l, and he proved it from hilchos succah. It states in hilchos succah that if there is space in the schach (called אויר) or if there is סכך פסול (not kosher schach) on the roof of the succah, the succah is pasul (see Shulchan Aruch 632:1-2). But there is a difference between a space and when there is non-kosher schach. Four tefachim of non-kosher schach disqualify the succah. An empty spot in the schach disqualifies the schach when there are three tefachim of air, emptiness. Reb Yechezkel of Kuzmir zt'l said that we can learn from this the great fault of emptiness, of not doing anything at all. Therefore, let us not be idle, especially during the holy days of Succos.
Shem MiShmuel (Shabbos Chol HaMoed Pesach 5673) writes, "Every moment of the holiday [chol-hamoed], one should be prepared and ready to receive the holiness that is coming down, non-stop."
16 Pele Yoetz (Chol HaMoed) writes, "Chazal (Yerushalmi Shabbos 15:3) sayלא נתנו שבתות וימים טובים לישראל אלא לעסוק בתורה Hashem gave Shabbos and Yom Tov to Yisrael so they could study Torah. Some people say they can't learn Torah because they are busy earning a living. If such a person uses his time well on chol hamoed, as well as the other times when he is off from work, and devotes himself to studying Torah, he will receive reward even for the times he was working. [Because by studying at every free moment and every opportunity he proves that if he didn't have to work, he would study Torah then as well.] He should be especially cautious to study Torah on chol hamoed, because chol hamoed are days of judgment. On Pesach, there is a judgment on the תבואה the grain crop, and on Succos the judgment is for water (see Rosh Hashanah 16a), and the gmar din is on Hoshana Rabba..."
The Mishnah (Avos 3:11) states, המבזה את המועדות... אף על פי שיש בידו תורה ומעשים טובים אין לו חלק לעולם הבא "Someone who disgraces the Yomim Tovim... even if he knows Torah and he has good deeds, he doesn't have a portion in Olam HaBa." Rabbeinu Yonah says that מועדות refers to Chol Hamoed. We must honor these days, to be very cautious not to desecrate them or to dishonor them in any way. Rabbeinu Yonah writes, "The Mishnah doesn't need to discuss Yom Tov because it is like Shabbos [and everyone understands that we must honor Yom Tov]. Rather, the Mishnah is talking about Chol Hamoed, when one says that Chol HaMoed doesn't have kedushah like the first days." Thinking that way is a desecration of chol hamoed.
The Chiddushei HaRim zt'l teaches that there is special kedushah on Shabbos Chol Hamoed. He explains that throughout the year, the kedushah of Shabbos comes and is created from our avodas Hashem that we performed during the weekdays before Shabbos. Shabbos Chol Hamoed is a very high time because its holiness is formed from the avodas Hashem that was done on Yom Tov and chol hamoed. This year, Succos, there is no Shabbos chol hamoed. But this lesson tells us of the great kedushah of the Shabbos of Shemini Atzeres.