Striking the Rock
במדבר כ,ח: קַח אֶּת-הַמַטֶּה, וְהַקְהֵל אֶּת-הָעֵדָה אַתָה וְאַהֲרֹּן אָחִ יָך, וְדִבַרְתֶּם אֶּל-הַסֶּלַע לְעֵינֵיהֶּם, וְנָתַן מֵימָיו
According to Sforno, there are three possible kinds of miracles: 1. Hidden miracle (dewfall); 2. Revealed miracle (ground opening); 3. Miracle which nature could never do (turning stone into water). HaShem wanted people to see the third kind through speech (דיבור). Moshe and Aharon decided to make the second kind because they did not believe HaShem would make the third kind for the undeserving Israelite mob.
Serpent of Brass
במדבר כא,ט: וַיַעַש מֹּשֶּה נְחַש נְחֹּשֶּת...
The Jews were guilty of Lashon haRa. Sforno states that the fashioning of the brass serpent aroused the Jews to examine their sin, bringing them to repentance. The staff was made of brass (נחשת) to underscore the link with the serpent (נחש) whose venom burns like slander.