Hi – we’ve written many times about watching the takeoffs from Boeing Field. The Museum of Flight started charging admission a few years ago to watch from its parking lot but as far as I know, you can still go hang out on the fence just south, next to where the jets are parked while here, to watch the pre-flight preparations.
People also park along Boeing Field in various spots – we went there earlier this week for their arrival – but some of those spots keep shrinking.
Good luck!
Tracy