Why did Par'oh say 'tomorrow' and not today?
Ramban #1 (citing R. Shmuel ben Chofni): The challenge 1 was based on the assumption that Par'oh would think that astrologically speaking, now was the time for the plague to go away, so he would say tomorrow. 2
Ramban #2: Because Par'oh thought that Moshe's request implied a short time-period - and did not incorporate 'today'.
Moshav Zekenim #1 (Pasuk 1): He thought that naturally, frogs can live up to a day outside the water, so he assumed that, by the following day they would all be dead, proving that Moshe was an imposter.
Moshav Zekenim #2 (in Pasuk 5): Par'oh claimed to be a god. He did not want to reveal that the plague pained him.
Riva: Par'oh said that he will send out Yisrael tomorrow. Refer to 8:5:2:5.