Boise i believe has been trying to get a new stadium built the past few years but have come up empty. I believe it was Tri City and Salem Keizer that were on the original chopping block but who knows now. Salems ballpark was opened in 1997 and Tri City in 1994, but Tri City also has an unusual layout in that it faces South rather than the more traditional East-Northeast. Because of which they built a giant wall or shade to block the setting sun to help with the setting sun in fans eyes. Everett was renovated in the late 90s. Vancouver and Spokane have old ballparks but have been modernized and renovated in recent years. Eugene and Hillsboro have the newest parks.
As for attendance, Salem and Tri City were at the bottom in 2019, followed by Everett and Boise. Vancouver and Spokane are the top two teams despite older stadiums. Boise, Tri City and Everett have the lowest seating capacities, while again Vancouver and Spokane have the highest.