Family Vacation in Seattle tips?

Hi all, we will be in Seattle for 4 days with our 5 year old boy. any
tips on fun things to do? my husband had suggested going to Mount
Rainier for a few days, renting a cabin and have fun exploring. i am
concerned the hikes will be too long for my son to hold his attention.
he loves "exploring" but not if he has to walk 5 miles! is this a good
park for kids? if not, do you have any fun suggestions for the area?
he loves to look for treasure, explore, find bugs, feed animals, etc.
i'd like something outdoors since it's nice this time of year over
there. we were in Victoria last year and LOVED it but we'd like to do
something different. Any tips on a fun vacation near seattle that a 5
year old boy would enjoy? thanks for any tips, Carla