The Portland Children's Museum is great, there is the Oregon Zoo, a free rose garden to explore and lots and lots of free trials at Hoyt that kids would love!
I am surprised the children's museum hasn't been mentioned. Also the Hoyt Arboretum.
There are walking/ hiking trails and a large play area as well as a big open grassy field for numerous activities to run and play.
Rose garden and a zoo
there is a zoo in washington park that is very great for kids. there is also a playground structure near the rose garden. i imagine the rose garden would also be enjoyable for kids if they like flowers
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