Why did Hashem instruct Aharon to strike the water for the first two Makos?
Rashi and Targum Yonasan: Because the water protected Moshe when his mother placed his casket in the river (See Sh'mos 2:4). Consequently, for him to strike it would have demonstrated a lack of gratitude.
Moshav Zekenim (on 8:2): Aharon was Moshe's Shali'ach, so he performed Makos from the land. Moshe himself performed what came from the sky.
What are the connotations of "u'va'Eitzim u'va'Avanim"?
Rashi and Targum Yonasan: It means water that has been poured into wooden and stone vessels.
Oznayim la'Torah (citing Midrash ha'Gadol): Water ctually oozed out of wood and stones. 1
Oznayim la'Torah: Because the Egyptians forced the Jewish slaves to place babies in the walls when they were short of bricks.