As you may know, the trip from Boise to Portland is roughly a 7-hour car ride, direct. But I have a different idea for you that will take you a bit longer, but what sights and scenery you will see!
First, you may want to plan a few hours in downtown Boise. It has come a long way. Then, I recommend you proceed on I-84 into Oregon (you’ll change your watch back 1 hour) until you reach Payette and Highway 26 heading West. From here, you will be traveling on one of the most scenic highways in the state along the John Day River. It will be flooding in June (no worries, but sign up for a rafting trip, if you like). There are charming towns along the way, an old Chinese apothecary museum in the town of John Day and bed & breakfasts and small lodgings to choose from as you head toward Fossil.
Also you will be passing through one of the most geologically significant regions in the state complete with fossils (into dinosaurs) and the pastel-colored hills of The Painted Hills, so you’ll want to take your time. There are innumerable places to stop for sightseeing and rock hounding and visiting the amazing Paleolands Institute along with other side trips to fossil beds if you’re interested. From the tiny town of Fossil you will wind out of the desert and join I-84 where you will meet the mighty Columbia River. Travel all along its borders and stop for lunch or dinner in Hood River, the windsurfing capitol where you’ll find lots of eateries, good coffee and sporty, adventuresome types. Spend a second night in this charming town, if you like. From here, it’s just 1 hour through The Gorge, another scenic highway, to downtown Portland.