N.F.
I'm a mom of a 9-year-old incoming 4th grader and a 5-year-old incoming kindergartener whom I just finished 2 years of co-op preschool with. I was the field trip planner for both those years, so I think I can help you. Here are some of our favorites:
Craven's pumpkin farm & Biringer farm; Everett's Children's museum; beach walk with the rangers in Edmonds (you have to book this one early); Creation Station in Lynnwood; ride a city bus to a public library for story time (Edmonds worked great for us); Shoreline fire safety center; tour a grocery store (we had a connection with an employee which really helped); Outback kangaroo farm in Arlington (totally worth the drive); WPZ brought a live owl to us for a visit (4-5 year olds did great with this); ride the Mukilteo-Clinton ferry over & back (contact them ahead of time about touring, and special school program rates).
These are the ones that jump out at me. Some of these cost money; we tried keeping the cost to $5 or less per kid, and very few charged for adults, too. Good luck with your planning!