We traveled to the Columbia River Gorge in late July, but we knew we would be hiking in the shady gorge and starting out at sunrise so we packed layers. We each had a variety of hoodies, long sleeve shirts and a light jacket. Quick-dry hiking pants were a must and turned out to be very comfortable and practical during our hikes (we wore Gander Mountain, Columbia and Northface brand pants like these). There was only one day (our last day at Mt. Hood and Trillium Lake) when I would have preferred a short sleeve shirt. The day was warmer, and the hike was more sunny; otherwise, we were always comfortable. We started out each morning with full layers and were able to remove them as the day got warmer.
Hiking shoes are a must! We wore our Keen hiking sandals (Little Britches wore tennis shoes and Crocs). My sandals were broken in and had hiked many miles in the Great Smoky Mountains, but Eric took new sandals. This turned out to be a bad idea, and he dealt with blisters on his heels during much of the trip. I always pack a small first aid kit so that came in very handy. Before I owned a pair of Keen hiking sandals, I assumed it was just a lot of hype because my feet are terribly finicky and very hard to fit. I've found my sandals to be the most comfortable pair of shoes I own, hiking or not. I highly recommend them, and I doubt I will ever wear another shoe for hiking. My Keen's are open sandals (not open toe), so I did wear socks (I know not very fashionable but I do not like cold feet) in early morning and removed my socks when the day got warmer or if we had to wade in water.
As far as accessories, our light jacket doubled as our rain jacket if needed. We also took sunglasses, which we didn't need very often due to the shady hikes. We packed hats but found they weren't necessary. We each had our own backpack for hiking (it also doubled as our personal carry-on for the airport). We all took bug bracelets (the hiking bracelets that smell like citronella) as a precaution, but we didn't have any issues with insects during our trip.