Welcome Xabier!
Forest Park is excellent for Dunn's Salamander, Western Red-backed Salamander, Ensatina, and Coastal Giant Salamander. Flip rocks near streams, replacing carefully. Dunn's will be just outside the water under rocks in damp areas, larval Giant Salamanders will be under rocks in the water. Western Red-backed Salamanders can be found under rocks next to the water or under logs and leaf litter on moist parts of the hillside near the stream. Ensatina are under logs and sometimes leaf litter or rocks, usually not near water. I've been able to find Dunn's and Giants in every stream in Forest Park. You might be able to find an adult giant under logs or other large debris just outside of the streams, but you'll have to get lucky. I've found about a dozen transformed adults in Forest Park but only within select very narrow habitat areas.
Macleay Park and Tyron Creek also have all three species easily. Other forested parks with streams might have those species as well.
There are Northwestern Salamanders in the Halburton area bordering Forest Park but I haven't seen them myself. They're in plenty of other places in Portland too, but the adults are not easy to find. Their breeding habitat is ponds and lakes especially in forested areas. Look under logs on slopes directly uphill of breeding habitat. Though you'll have to get pretty lucky to find a Northwestern Salamander, you might find some Long-toed Salamanders or Rough-skinned Newts in the process.
There are a couple slender salamander localities on the east side of the metro area, and more in the gorge. I'll let you do your own research on that one.
I have brief wildlife guides on my website that do a comprehensive job of showing exactly what species are found in our area. However, since my guides are specific to Columbia County, they're missing a few species that you can find east of Portland (Oregon Slender, Cascade Torrent, and Larch Mountain).
https://wildcolumbia.org/
