The last hope is with Congress. They still have the power to stop this. All they have to do is impeach Trump for treason before he is inaugurated.

Edit: To clarify, they have to CONVICT on impeachment. An impeachment alone is not enough.

I can tell that some people are panicking, so I thought I'd provide one last dose of hopium.

It hinges on the certification. Congress can still reopen the joint session and file objections. It's about the technicalities and semantics of "ascertainment". Congress has tentatively verified the authenticity of the 50 certificates and Harris "notarized" their assessment. Ideally, objections happen during or before Jan 6, but I see nothing that would preclude a valid objection after that. Hell, they should still be able to object to a cert even after the inauguration, but I won't waste time speculating on that right now.

Obviously the objections would have to be made, which is where the Democrats come in. It probably won't be Republicans signing the objections, but we have more than enough Democrats in both houses to sign them. That opens the debate in each house, and then they vote to "sustain" the objection, which I think would trigger a recount of the electoral votes minus the affected certs. The affected certs would get bounced back to their respective States. Then the States can amend them through rapid legislation and send them back. Then another recount occurs and Harris once again notarizes it.

It would be like when the Vatican bishops meet to choose a new pope. We'd all be sitting on the edge of our seats waiting for the white smoke.

Do I believe this will happen? Nope. But you can, if you like. Whatever happens or could happen, we cannot hope for something illegal/amoral/unethical. We're better than that.

Edit: Here's the Archive's FAQ that may be informative. https://www.archives.gov/electoral-college/faq