The spa and hot tub is good at Bonneville and hugely popular with eastern Europeans from Portland area. I use it for 'Day use' during the off season. It is set back off the road and not a 'view' property like Skamania Lodge (which I go to enjoy the view / library / fireplace). Often go go to both places the same day. Have heard the lunch menu is good at Bonneville HS, but I do Stevenson eateries for supper. (The Fish House on the water front was decent last I tried, ~ 5 yrs ago)
Walking Man Brewery is supposedly one of the best in PNW (along the tracks across street from fire / ambulance station) I like the Salmon Chowder at Big River Grill, others like the mex food (which you probably have better in tri-cities)
The setting of the outdoor hot tubs is desirable at Skamania Lodge, but they don't have 'hot springs' (which is something I frequent.) Enjoy relaxing in the multi story lobbies of each (Bonneville and Skamania). There are many private 'public' areas to enjoy at Bonneville. It is great when they are having winter concerts in the lower lobby.
If you are into more vistas, consider the new Cape Horn Trail
Cape Horn Loop Hike (that is where I live). For the shorter version of the hike, you can park and hike north from the upper crossing of SR 14 and see the view, then return, and get in your car and drive down the Cape Horn Landing Road. The whole loop is nice too.
If it is really hot (predicted) Do some waterfall hikes on the OR side, just go across to Cascade Locks and west on I-84 till you get to Historic Highway (Bonneville Dam exit), then on west toward Multnomah Falls (Horsetail Falls is a nice short hike). The Gardens at the Bonneville Hatchery (OR Side) are very nice, and there shouldbe a couple 7' sturgeon there.
Bring your credit card, as the Pendleton Store in Washougal, WA is having a pretty big sale. (bigger than usual as some of their factory store storage is being demolished)
Also bring a cooler and take home a few Steelhead, the NA's are selling them just east of Bonneville Dam. I usually pay $3/# or nearby.