And why did Pharaoh say 'tomorrow' and not today?
Ramban #1 (citing R. Shmuel ben Chofni): The challenge 1 was based on the assumption that Pharaoh would think that astrologically speaking, now was the time for the plague to go away, so he would say tomorrow. 2
Ramban #2: Because Pharaoh thought that Moshe's request implied a time-period (and did not incorporate 'today').
Moshav Zekenim #1 (verse 1): He thought that naturally, frogs can live up to a day outside the water. Seeing them live to the next day proved that it was a miracle.
Moshav Zekenim #2 (verse 5): Pharaoh claimed to be a god. He did not want to reveal that the plague pained him.
Riva: Pharaoh said that he will send Yisrael tomorrow (refer to 8:5:2:5).