This is a late response, but for activities, check out the local libraries. Tigard Library is fantastic and has a ton of children's activities and story times.
The nearby recreation centers have indoor play areas, which are especially nice on our rainy days.
Try www.kidzlist.com for happenings around the Portland Metro area. We have a nearby Children's Museum, Zoo, Forestry Center (some of these can be free with a Cultural Pass that can be checked out at the libraries).
There is Moxie Moms and some coffee shops with play areas where you might meet other moms/kiddos.
Hope you are settling in okay and it makes for a busy holiday season for you. Good luck to you, I too am in Tigard with a 5 yr old girl and 3 yr old boy. Let me know if you need to have some adult company.
There are a couple of great cooperative preschools in the area. We really like Touchstone Preschool at 72nd and Hwy 217 in Tigard. It is a bit pricey, but a great, well-rounded program. They offer PE, Music, and Spanish on top of classroom time. My kids love it there and the teachers/staff are all very caring.