Most big charities and thrift stores won't accept car seat donations because of liability issues. However, you can still find local charities or community members who will be happy to have your in-good-condition car seat. If not, your best bet is saving your seat and bringing it to Walmart or Target on a trade-in day.
What do I do with old car seats near me?
Puget Sound area parents and caregivers can bring car seats and boosters to the Total Reclaim recycling facility at 2200 6th Ave.