I had to check what I wrote on Trip Advisor about your trip so I would not repeat stuff (-:
I will try to cover the south Okanagan
There is a good birding book for the area http://southokanagannature.com/south/ok ... /113-Books
It is a great layout; in full colour; it has good instructions of how to get there. It is great value.
Most of the birding spots are along paved roads so it will not be a problem with a rental car as most will not allow travel on gravel roads.
You may find that at Vaseux Lake in August the walkway will be overgrown; signs about poison ivy and lots of bugs, but across the road which shows up at Dutton Creek Road on google but sign says McIntyre Creek Road you should find some California Bighorn sheep. There is a flock of chukar in the rocks but they are very elusive. The road is not paved but in good condition if you only go up to the first cattle guard. https://goo.gl/maps/JrLKov1Yzzx
and streetview https://goo.gl/maps/3mBJTUWJZXU2
Haines Prov Park in Osoyoos usually has something.
The White Lake Road has the Dominion Radio Observatory and a dry lake bed where there are meadowlarks but really hard to spot in the sagebrush. There are 3-4 access roads to get to White Lake the one from Okanagan Falls that goes by the See Ya Later Ranch winery with a great valley viewpoint has a Green Lake just past the winery. It usually has birds but not sure in late August has it could be dry or little water. There is only one place to stop along Green Lake https://goo.gl/maps/SitA42uAPmt
I keep track of sightings with monthly albums in Flickr - https://flic.kr/s/aHskhn7gSG
The Osprey have nests on man made platforms located throughout the valley; the one at the Penticton Airport is one that is easy to get at and they are used to the traffic.
For the Kelowna area it will depends on what you decide about flying into Calgary or into Kelowna. Generally I default to Keith for Kelowna sightings https://www.flickr.com/photos/8666250@N02/
Keith also follows this forum.
I did a route between Vernon and Penticton for birders from TO who are coming in June which I can share if you decide to start in Calgary and do not want to stay in Kelowna as you mentioned on your TA posts.
I have a trip to the coast planned for next month. What i found is that with 3 full days if I stay in White Rock or Tsawassen i can cover: the White Rock Pier; Blackie Spit; Boundary Bay; the Heron Rookery at the ferry terminal; George C Reifel Centre and Steveston all places that I have seen from following locals posts and their pictures on Flickr. So maybe we both will benefit if we get comments about the plan that I have. https://goo.gl/maps/EN528coXdER2