I can give you lots of places in and around Seattle if you're interested, but not on the way. Maybe check your library for "Dog-Friendly Hikes in (State)". I've got one for WA and AZ, I'm sure they have them for other states.

Thanks! My son and daughter-in-law (the dog owners) are settling in Seattle, so any Seattle info would be wonderful. We'll look for those books...

Seattle has some great off-leash dog parks, but even the ones that don't have specific areas are good for leashed pets. My favorite off-leashes are at Marymoor, Westcrest, Magnuson, and Golden Gardens. In-city hikes can be found at Discovery and Lincoln parks, and Greenlake is a dog-walkers (and joggers/bikers) mecca.

Slightly farther out, you can go north to the Mountain Loop Highway for many hiking trails (Mt. Pilchuck and Lake 22 are favorites) and east to Tiger Mountain in the Mt. Baker -Snoqualmie forest.

They will find that Seattle generally is a great place for dog people, bonus is they may find being out and about with the pup a good way to meet people.

More of the same, but quite useful...

http://www.dogfriendly.com/
http://www.petswelcome.com/

While National Parks don't allow dogs on trails, National Forests (with a few exceptions for particular trails) do. So, look at your route, and look for the surrounding public lands on a map. You should be passing through or by quite a few national forests. Each NF has a website with recreation opportunities, and you can find hikes there.

I'm unclear about your route--I don't know when you are leaving I-80 and heading to the PNW--so I can't mention specific national forest areas.

Oh, okay, I see the route you're taking. Many national parks (such as Yellowstone) are surrounded by national forest, so you'll be able to find doggish hikes around there. If you end up staying around Ketchum/Sun Valley in Idaho, there are tons of NF trails there--look at their tourism website. If you're passing by any state parks, you can check websites to see dog rules.

1. When you are in bear country, you are going to want to think real hard about taking your dog on the trail.

2. Our cairn terrier was a great hiker despite his short legs. He earned his stripes in the White Mountains. We had to lift him over some of the steeper rocks, but there was nothing he wouldn't try. But he sure did sack out in the car on the way home!