In the Hagada of Peasach it says "even if he brought us to Har Sinai but didn't give us the Torah it would have been enough." What would be the purpose of going to Har Sinai if we didn't get the Torah? How would that be enough?
One popular explanation:
When the Jewish People reached Mount Sinai, the Torah (Ex. 19:2) describes their encampment in the singular form - ויחן שם ישראל נגד ההר. Rashi, citing Mechilta, states that this means that they really felt unified, "as one person, with one heart."
That unity among Jews, then, was by itself worth reaching Sinai in order to achieve.
Aside from the other excellent answers listed here, two points:
1) Dayeinu does not mean that it would have been enough for us, as in it being an end in itself, but rather than it would have been sufficient cause for us to give praise to Hashem. See here for further elaboration. Each step in the process was wonderful and deserving of our praise.
2) This was a fulfillment of a Divine promise in Shemot 3:11-12:
יא וַיֹּאמֶר מֹשֶׁה, אֶל-הָאֱלֹהִים, מִי אָנֹכִי, כִּי אֵלֵךְ אֶל-פַּרְעֹה; וְכִי אוֹצִיא אֶת-בְּנֵי יִשְׂרָאֵל, מִמִּצְרָיִם.
11 And Moses said unto God: 'Who am I, that I should go unto Pharaoh, and that I should bring forth the children of Israel out of Egypt?' יב וַיֹּאמֶר, כִּי-אֶהְיֶה עִמָּךְ, וְזֶה-לְּךָ הָאוֹת, כִּי אָנֹכִי שְׁלַחְתִּיךָ: בְּהוֹצִיאֲךָ אֶת-הָעָם, מִמִּצְרַיִם, תַּעַבְדוּן אֶת-הָאֱלֹהִים, עַל הָהָר הַזֶּה.
12 And He said: 'Certainly I will be with thee; and this shall be the token unto thee, that I have sent thee: when thou hast brought forth the people out of Egypt, ye shall serve God upon this mountain.'
I'd have to find the source, but one of the answers I remember learning is based on the Talmud (Makkot 23B-24A).
There (also brought in this answer), the Talmud tells us that the verse (Devarim 33:4) "תּוֹרָה צִוָּה לָנוּ מֹשֶׁה מוֹרָשָׁה קְהִלַּת יַעֲקֹב", hints to the 611 commandments that were given to us by Moshe. "תּוֹרָה" is the numerical value of 611. The first two commandments, "Anochi" and "Lo Yiheye LeCha" were said by G-d.
So we are saying, if G-d had only given us the first two commandments on Mount Sinai, and not given us the other 611 (numerical value of Torah), it would have been enough.
This fits in nicely with the Ba'al HaTanya's statement (Tanya Chapter 20 and Chapters 24 - based on the Shaloh beg. Parshat Yitro and Zohar II, p. 276a.) that "Anochi" inlcudes all the positive commandments and "There shall be no other gods before you" includes all negative commandments, since doing a Mitzvah affirms G-d's unity and transgressing any commandment is essentially denying G-d's unity.
In the Haggadah Shel Pesach Im Likutei Ta'amim U'Minhagim, The Lubavitcher Rebbe gives two answers:
"If He would have brought us to Mount Sinai...", about which it says (Devarim 5:21) "Behold, the Lord, our God, has shown us His glory and His greatness" (and as we say in the liturgy for Mussaf on Rosh Hashanah, "And You revealed Your glory in a cloud on Mount Sinai"), "...and not given us the Torah, it would have sufficed us"
"If He would have brought us to Mount Sinai..., where we would have heard the 10 Commandments, ".....and not given us the Torah..." afterwards (referring to the Luchot, Torah and Mitzvot), "it would have sufficed us."
HaSeder Haruch (vol. 3, Iyunim Behagada pg. 414) collects ten explanations (some of them may overlap answers already posted, but I nonetheless bring the whole list for sake of completeness and because they are well sourced):
The Gemora (Shabbos 146a, Avoda Zorah 22b) states that when the Jews stood before Har Sinia "Paskah Zuhamasam Miyisroel" (Machzor Vitri, Orchos Chaim, Kol Bo, Shu"t Haravad 11)
In the days of separation before we received the Torah, the Jews were given over mystical secrets of Torah (Mikesem pg. 138)
"If He did not give us the Torah" refers to the Asseres Hadibros which were given directly by Hashem. They could have otherwise come via Moshe Rabeynu, like the rest of Torah (Rashbam, Orchos Chaim, Kol Bo, Abudraham)
Through being brought close to Har Sinai the Jews merited to have fear of Hashem, as it is written (Shemos 20:17) regarding Har Sinai: "And in order that His awe shall be upon your faces, so that you shall not sin." Thus, the Gemora (Nedarim 20a) writes that if one does not have "בושת פנים" (humility) this is proof his ancestors were not present at Har Sinai. (Rashbatz, Hagadah Mibeis Levi in name of R' Chaim of Brisk)
"If He did not give us the Torah", means the Torah in its entirety with all the details and specific commandments, but rather only gave us a few commandments (Zevach Pesach)
The Zohar (Shemos 82a) states that when the Jews stood before Har Sinai, Hashem opened up the skies and we saw the "Markeva Elyona". This would have enabled us to fulfil all the Mitzvos, as they are all dependant on the "Markevah Elyona" - just as Avraham kept all the Mitzvos because he comprehended the "Markevah Elyona". However, in the kindness of Hashem, He commanded us to observe Torah and Mitzvos, for it is preferable to keep when obligated. (Hagadas Toldos Adam by the author of Chayei Adam)
Even if we would not have received the Torah, we would still have derived a lesson of humility from Hashem's choice of Har Sinai - the smallest of mountains (see Sotah 5a) (Eish Das)
When the Jews stood before Har Sinai, the Torah used the singular form of the word to tell us that they encamped ("ויחן" rather than "ויחנו"). Chazal explain that the Jews at that time were "like one person with one heart," and this in itself was a great achievment (Ksav Sofer, Droshos vol. 1 to Shabbas Hagodel 5419).
Even if we did not receive the Torah, Hashem would have given us the seven Naochide laws again at Har Sinai (Hagadah Mibeis Levi in name of R' Chaim of Brisk)
The Gemora writes (Sanhedrin 59a) that a gentile who learns Torah is liable to be executed because Torah is an exclusive heritage to the Jews. We could explain that "If He did not give us the Torah", means if He did not give the Torah exclusively to us but also allowed gentiles to learn it. (Hagadah Mibeis Levi in name of R' Chaim of Brisk)
Rav Leuchter in his second shiur on the Hagodo says that at the time of Har Sinai “posku zuhamosom” Klal Yisroel went back (admittedly for a short time) to the state of Adam before he sinned. That would have been enough (reason to be grateful to Him).
R. Shlomo Kluger in his explanation of the Haggadah here says that even if Yisrael had not been commanded the Torah they would have known it by themselves, just like the patriarchs knew the whole Torah. As the Alshich explains, when Yisrael came to Mount Sinai the spiritual impurity that was in them from the sin of Adam HaRishon went away, and therefore they were able to sense the Torah by themselves.
We still would have been a nation, and then could have formed our own set of ethical laws. That seems to have worked out well for the Greeks around the same time. B"H we got the Torah instead, and have since long outlasted our secular contemporaries.