Saint Martins Mineral Hot Springs is a 1 mile moderately trafficked out and back trail located near Carson, WA that features hot springs and is good for all skill levels. The trail is primarily used for hiking and walking and is accessible year-round.
The hotel is turn of the century and the mineral water is piped into old claw foot tubs. Great experience, but if you want something more luxurious I would suggest Bonneville resort west of Carson.
I am not sure if these hot springs are still accessible, but when I used to go, you had to drive under some huge power poles onto private property. When you arrive at the parking lot there is a $5 fee to park. The hike to the hot springs is about half a mile up stream. There is a rugged trail that hugs the river and its not exactly kid friendly. Once you arrive you will see some naturally formed pools less that 5 feet from the raging river. I assume these pools are underwater in winter, spring and fall. The pools are pretty hot, but not scalding. Human presence is obvious by burned candles and the occasional bottle laying around.