What were all the good things that Avimelech claimed to have performed with Yitzchak?
Ramban: He was referring to the fact that (a) They did not touch him or his wife; (b) They were careful to guard his property, and (c) They sent him away with all his wealth intact.
Oznayim la'Torah: In the eyes of the Nochrim, they did the Jew 'a great kindness' when they only drove him away - body intact - stealing the land which he had tilled by the sweat of his brow. They robbed him of the wells that he had dug with much toil - and sent him away 'in peace' - without breaking his bones. That is what a Nochri calls 'only good!' 1
Ohr ha'Chayim: Kings usually help their residents for their own benefit - (a) to be able to borrow from them later if needed, or (b) having wealthy, esteemed residents aggrandizes the kingdom. We did not intend for either. We sent you away!
Malbim: It is the custom of kings to take the wealth, or part of it, from strangers. We did not take from you! This is evident now that we sent you, and did not touch (take from) you. As long as you were in our land, a king does not mind if a subject gets rich, for 'what a slave acquires belongs to his master.'
Ha'amek Davar: It was good for you that we sent you away, lest people loot from your property. Even though the decree not to touch (do business with) you was due to hatred, it did not harm you. It merely withheld benefit.
Da'as Zekenim: A parable for this is a lion that had a bone stuck in its throat. When it announced that anyone who can remove it will be amply rewarded, a crane volunteered to do the job. It stuck its long neck into the lion's neck, pulled out the bone and asked for its reward. To which the lion replied, 'I sent you away in peace without killing and eating you when you placed your head in my mouth - is that not the biggest reward one could ask for? And you still want a reward?!' That is precisely what Avimelech said to Yitzchak, 'We did you a great kindness by sending you away in peace - without harming you, as we normally do to strangers!'
What did Avimelech mean when he concluded, "Atah 'Atah Beruch HaSh-m!" ('You, now, blessed of HaSh-m')?
Rashi and Seforno: He meant that Yitzchak should do them no harm, just as they had done him no harm.
Ramban, Targum Onkelos and Targum Yonasan: He meant that although now Yitzchak was blessed by HaSh-m and was untouchable, that would change dramatically should he do them harm; in which case Yitzchak would be forced to return to them, and they will deal with him accordingly.
Rashbam: He meant that Yitzchak, for his part, should reciprocate by entering into a covenant with them and send them away in peace 1 (like they did with him).
Oznayim la'Torah: Avimelech meant that they wanted to enter into a covenant with him, which Yitzchak was guaranteed not to abrogate, since he was currently blessed - and abrogating it would cause him to become cursed.
Ohr ha'Chayim: Before, you were in our land, which has a Sar (angel) over it. Now there is no intermediary between you and HaSh-m; He directly blesses you.
Refer to 26:28:1:6.
Ha'amek Davar: Now you are blessed, but we are not. This is a Siman for future exiles - after nations expel Yisrael, they understand that they harmed themselves.
A request with which Yitzchak complied (as stated in 26:31).
Why was the stipulation not to do evil?
Malbim: Also the Bris with Avraham was not Batel, for it did not obligate bestowing good, just not to harm the other.
QUESTIONS ON RASHI
Rashi writes: "'We did not touch you' - when we told you, 'Leave from among us' (26:16)." How does Rashi know that it refers to that instance?
Gur Aryeh: Avimelech said, "Just as we have done to you only good;" if so, they certainly had not [claimed to have] done him any harm! Rather, the intent was, 'As long as you dwelt among us, we did for you only good; and even when we sent you away, we did not do you any harm."
Rashi writes: "'You' - You, as well, [please] do for us the same." Is all this derived from the word "you"?
Gur Aryeh: Rashi is telling us what to interject into the verse - "You, [as well, please do for us the same] now, O blessed one of HaSh-m!"