If you have time to waste, you can seek (and probably be denied) extra compensation. Many people think they can get more money out of airlines than airlines are willing to give out. A lot of compensation passed out is already voluntary by the airline (like that you've already been offered). If you have higher status with United, sometimes that might get a little extra consideration. An award ticket from AC? Good luck with that

"90 days", IMHO, seems like 90 days to accept or it vanishes.