The alternatives are fairly simple: you can either increase revenue or decrease benefits.

Increasing revenue (number games aside) would come from productivity gains or additional working population.

Decreasing benefits would come from cutting the payout or increasing qualifying age.

Honestly, taking Medicare out of the picture as a different problem that requires different solutions, Social Security should have been indexed to life expectancy from the beginning. It was never practical to build up a surplus that would be of sufficient magnitude to address demographic imbalances over decades.

Grandfather people in the program into the current rate, apply a sliding scale to people close to retirement (only fair, so their expectations don't drastically change), and make the hard decision.