Check the library out. They have story hours, etc. Find yours here: http://www.50states.com/oregon/libraries.htm
If you have the money, join a gym with a Kid's Club. I don't know where you live, but Club Sport has a great program. Good way to decompress for a couple hours and have some adult time while your little one plays with other kids.
The Children's Museum and OMSI have great discovery play areas (and you can become a member if you really like them and plan to visit frequently). Same goes for the ZOO, but the others are really good for rainy days.
There are kids play gyms like The Little Gym or Gymboree or places like Westside Gymnastics Academy has a good program for 3-5 year olds.
Have fun!