Can a day trip from Seattle to the San Juan Islands be done?

Absolutely. There are several ways you can go. You can drive from Seattle to Anacortes (either by private car or shuttle bus) in about 2 hours and then take the ferry to the San Juans. The ferry ride to Friday Harbor takes about an hour with no stops (longer if it stops at Lopez and or Orcas). In the summer, the ferries leave as early as 4:30 AM and return as late as 10 PM. The ferry lines can be fairly daunting, so plan accordingly. You can also fly out of Boeing Field or out of Lake Union on a seaplane. The flights take anywhere from a half hour to an hour depending on which aircraft and destination you choose. If you google San Juan Islands, you will find a number of exellent websites that offer additional information. It's a beautiful area, have fun.