Fun question! There are a few places in the Willamette Valley that are known as being great for rockhounding.
The areas surrounding the South Santiam River, the Calapooia River, and Quartzville creek (all of these are about an hour southeast of Salem) are popular with rockhounders and folks who pan for gold as well. Also, check out this website and this BLM brochure for the Quartzville Mining Corridor.
Another fun place for rock-lovers is called Holleywood Ranch. It’s located in the hills outside of Sweet Home (again, southeast of Salem, near to the other locations I’m mentioning). It is a piece of private land that happens to have huge deposits of petrified wood and rocks. They’ve opened it up to the public for folks who want to come pay a small fee and dig for their own petrified wood. It’s a fascinating place!
For more information, the Bureau of Land Management provides great resources on rockhounding in Oregon and might be able to help you track down more locations.