On the way, if you are going to cut south to Plush for sunstones (recommended) you aren't far from Virgin Valley across the Nevada border for opals. There is a nice free campground with hot springs there for tent or RV camping if you get there early to nab a spot, though all the digs are fee-only (fairly pricey - $40-$100+ per person, but hey they're opals). You can next go east on US140 to Denio, and there are some digs around there before you head south to Winnemucca, then east on I80 to Salt Lake. On the way, you could take a side trip to the Lucin area for variscite, but will have to have some good luck to get anything on unclaimed land - interesting and seldom-visited area, though.
On the return, go north on I15 to link up with I84 west. If you have a bit more time and a rugged vehicle, you can cut over at Caldwell to Homedale and from there get to the Graveyard Point area and explore the unpaved roads to the west. Have a good map and skip dirt roads if its been rainy. You'll want to work your way north from there back up to Vale or Harper, and there is good material all over. If you don't have time and/or don't trust your vehicle on rough roads, skip Graveyard and just get off I84 on US20 at Ontario and head west. I'd suggest the picking up a current copy from your local bookstore of either Rockhounding Oregon or Oregon Gem Trails. Either will give you plenty of locations along US20, though Glass Buttes and the Hampton Butte area are the most convenient along the highway, especially for 2WD. Stay on the main, graveled tracks at Glass Buttes and Hampton to avoid getting a tire puncture. If you are returning to the Portland area, head west to Bend, then north on US97 to Madras. It is a brief sidetrip to Richardson's for some really easy thunderegg digging before heading home (it is a fee dig, but cheap unless you start to get tempted by the rock shop offerings). Alternatively, and if you have more time, you can by-pass Bend by cutting north just east of Millican on Hwy 20 and go to Prineville and do some free digging at Maury Mt. and thunderegg beds - both east of town - before heading home.
You could reverse that, too.
