Prologue: So what exactly is the connection between the two anyway?
Rashi begins the study of Parshas Behar with his infamous question concerning the parallel between the fact that the Jews were encamped around Har Sinai and the laws of shmitta which they were about to learn. Rashi asks the obvious, "Mah Inyan Shmitta Eitzel Har Sinai?" Why is the study of Shmitta connected to Har Sinai? Rashi explains that at Sinai the Jews received all aspects of Torah including those they would not utilize right away. Still, despite lacking immediate practical significance, these laws are also a part of one complete Torah that was received at Har Sinai.
The Brisker Rov (Chiddushei HaGriz, cited and expanded upon by Rabbi Dr. Y. Nissenfeld) explains that there is a deeper connection within the words of Rashi. Shmitta is a tough Mitzva. Its laws are intricate and, on the surface, boring to the plain reader. Maamad Har Sinai, on the other hand, was full of excitement. The drama of the receiving of the Torah had a flair whose Possukim scream for elaboration. One could assume that the shmitta laws need not be studied with the same enthusiasm as the Mamad Har Sinai section of Torah. but the opposite is true. The Inyan of Maimad Har Sinai should be studied with the same care as the precision we place on Halachic matters. And the intricacies of Halachic matters should be studied with the same enthusiasm we approach the textual descriptions Maimad Har Sinai with. This is the true understanding of the connection of the Inyan Shmitta and Har Sinai. Although the aspects might be different, it is truly one Torah, and the excitement and tediousness we use in studying the different component parts should be used all the time.
Legal Lego?: Lego building on Shabbos
The Mishna in Shabbos (102b) notes that any sort of building that is done on Shabbos can make one Chayav for the Issur of Boneh. The Gemara (Shabbos 122b) determines that the main Issur involved in Boneh is when building is done to something that is attached to the ground. A Machlokes ensues in regard to whether there is a concept of Boneh by Keilim.
The Yirushalmi (Shabbos 22:3) notes a Machlokes as to whether temporary building has the status of Building on a DeOraisa level or not. The Bavli (Beitza 32b) seems clearly aligned with the opinion in the Yirushalmi that a temporary structure isn't considered a building on a Deoraisa level but is nonetheless prohibited on Shabbat from a Rabbinic standpoint. The Chasam Sofer (Shut Siman 72) and Shut Har Zvi (39 Melachos "Tel Harim" -Meleches Boneh) both agree that this is the proper opinion L'Halacha. Rav Zvi Pesach Frank (Har Zvi, ibid) cites proof from Moshe who erected and took down the Mishkan every day of the 7 days of Miluim. The Shabbat had to be one of those 7 days. The Shevisas HaShabbos (Klallei Melacha SheEinah L'keeyum) explains that if the building was never able to stand forever, then it certainly is not considered a Binyan and can't be Chayav for Boneh on Shabbos according to all opinions. This concept appears to be based in the Shulchan Aruch as well (Orach Chaim 314:10).
This whole discussion becomes relevant in regard to the children's use of Lego/Duplo type building toys on Shabbat. Rav Ovadiah (Shut Yichaveh Daas IV:55) and Rav Waldenberg (Shut Tzitz Eliezer Xiii:30-31) allow children to build with Lego because one does not plan on maintaining the built structures on Shabbat. Shmiras Shabbos (16:18) agrees provided that the child does not need to attach pieces with screws or the like.
Rav Shlomo Zalman Auerbach ztl. (See Shmiras Shabbos footnote 53) still feels that adults should not play with the Lego. This is not because of the formal Issur of Boneh. Rather Rav Shlomo Zalman maintains that when building with Lego, one makes many different shapes and styles. Each time a new item is built, it is like Binyan Keli MeeTicheelaso even if the work is effortless. Still, others (Rav Ovadiah and Rav Waldenberg, ibid) hold that there is no problem with building and playing with the Lego toys on Shabbos.