Here’s a Map of All the Hot Springs in Washington State
Hot springs are one of natures beauties. I mean, it's a water naturally heated by Earth. Amazing! Hot springs have plenty of benefits but the trouble is finding them. Here's a convenient map of all the hot springs in Washington state.
Unfortunately for Yakima, it looks like most are on the west side of the state. We do have a couple in eastern Washington but they're very far north near the Canadian boarder. Also, keep in mind that visiting these at a part will require a Northwest Park Pass.
1889mag.com lists some of the better ones to check out. Some of the commercial ones require a reservation.
The guide from Seattle Mag lists a few good ones, some in Washington while some are in Oregon. They also remind you of etiquette to keep them open to the public by asking you don't trash the place.