Ok this is what I did in June in 2 days, I suggest more time because we were very rushed and barely had any sleep.
Take i5 all the way to hwy 433, merges into hwy U.S. 30W which then becomes 101, drive all the way down to Cannon Beach and hang out there for a few hours, make sure to check out the Haystack Rock. Then take Hwy 26 to Portland (if you like donuts check out Voodoo Doughnut, it's pretty cool and open 24/7, we went there at like 2am lol), after that (next day or whenever) take i5 to hwy 569 and then 126 to Florence to check out the Sand Dunes and Sea Lion Cave.
Here's my route on google map.
S Kelso Dr to OR-126 W/Rte F - Google Maps
Picture I took at Cannon Beach.