/u/unicorn1234 already commented about Easter. It's likewise difficult to find evidence that Christmas was a co-opting of a pagan festival. The most common suggestion is Sol Invictus, but the earliest attestation for that is the same document, the Chronicon of 354.

Overall, it's possible that Christian leaders intentionally co-opted pagan holidays, but there's a decided lack of evidence to secure the case. No early christian documentation suggests it. And, so far as I know, no early opponent of Christianity accuses them of it.

"The most common claim I've heard is that Easter was some kind of spring fertility rite until the Christians came along and made it all about the resurrection of Jesus."

I've heard it too, but it's pretty much entirely wrong. The Christian festival commonly called 'Easter' in the English language is also:

a. Only called 'Easter' in English, and a few cognates in a couple of other languages. In almost every other language it's some variation on the word 'Pascha' for Passover (and even in English, 'Pace eggs' is an alternative term used for Easter eggs in parts of England)

b. The dating for Easter is based on the Jewish calendar, as Jesus was in Jerusalem on the season of Passover

c. The controversy surrounding how to date Easter exactly was something of a controversy in the early Church in England, with Celtic and Latin churches using different methods or calculations for dates (the Synod of Whitby in 664 A.D being one attempt to remedy this). At no point was any pagan date or celebration ever suggested.

d. There is limited evidence for many 'pagan spring fertility festivals'. We have a few examples from among the Romans, because their calendar is well documented - but none of them corresponds to Easter. Part of the confusion comes about because many Neopagans celebrate a spring sabbat called 'Ostara' on the equinox, and some confuse this for an ancient pagan holiday (in addition to all this, some pop-Neopagan websites will claim that Easter is a Christianisation of this holiday). The problem here is that Neopaganism is not the same as ancient paganism (though it does draw inspiration from it), and in this specific case, the spring equinox observance called 'Ostara' only began among Neopagans in the 1970s (originally, they had only four holidays, while four more - including Ostara - were added in later; and even today, in the Wheel of the Year, Ostara is considered a 'Lesser Sabbat' while others such as Samhain or Beltane are 'Greater Sabbats', because those were the originals).