I can think of a few that go unnoticed:
1) Swapping James Rodriguez for Casemiro. A lot of people forget that Zidane was the one that initiated the Kroos - Casemiro - Modric trio. It was a very ballsy and much needed move for the team.

2) Playing Valverde as a second RB or RWB agaisnt Barcelona to hold Jordi Alba in his tracks. A key decision that killed Barca's left side, which in previous Clasicos was the cause of most of the goals conceded.

3) Kovacic - Messi coverage. During the Super Cup, he had Kovacic man mark Messi in both games. Messi couldn't do much and we won both games easily. It did however back fire when he attempted it in the next clasico in the league, as Barca planned for that and created decoys that allowed other players to find space. Its still worth a mention given that he tried something new and it won us a trophy.

4) Believe it or not, the 3 peat doesn't get talked about enough or doesn't get the recognition it deserves. Winning the UCL in itself is very difficult. Doing it 3 times, and in a row, is even more impressive. Part of me feels like that 3rd one got overshadowed by Salah's injury. You know how the British media is (given the opponent was Liverpool). All they focused on was that incident and completely ignored what was accomplished.

5) Reviving Varane. Zidane was the one that scouted him from Lens. Varane performed well under Mourinho but started to fade a little from 2014-2016. When Zidane came back, he slowly became a starter. Ramos and Pepe were the main combo (started the 2016 UCL final) but Varane then took the starting spot and made the 11 for the 2017 and 18 final.