Constitutional options:

• There are numerous cases headed toward SCOTUS. This was tossed on standing. That doesn't necessarily mean that the others will be. That said, only about 1 in 100 cases get heard by SCOTUS.
• Legislatures step up
• Electoral College goes faithless (not gonna happen, but included for the sake of being complete)
• Senate and House step up

There are the options that I am aware of, overlooking the more... interesting possibilities for next steps.