#2 exists (Alaska lounge sells day passes for $50/pp, $25 if you have their credit credit). Value is subjective, but I would not personally consider it worth the money.
Day passes are no longer available in the Seattle Alaska Lounges. So #2 is off the table.
The Museum of Flight is amazing and would be my first choice for a short layover.
The downtown option is more iffy because you won't know about traffic problems until you are caught in them.