Hi Jack,
I would base yourself in Osoyoos. It will be hard to reach certain areas for regional specialties, but it puts you in a good position to pick up the majority.
For example, if you can rent a bike and do about 30 km in a day, you could travel to Road 22 and pick up birds like Yellow-breasted Chat, Bobolink, Least Flycatcher, Grey Catbird, and some sagebrush/open country species (e.g., Vesper Sparrow, Western Meadowlark, and Western Kingbird) if you head just past onto Black Sage Rd. On the way back, you could visit the Osoyoos Desert Centre but unfortunately you'll be just a touch too early for Behr's Hairstreaks. Their flight season starts up in June.
Additional locations within 20 km of Osoyoos include Kilpoola Lakes (good butterflying and birding), Mount Kobau (great butterflying up a nice elevational gradient, including higher elevation sagebrush that could have Sheridan's and/or Bramble Hairstreaks, Brewer's Sparrow, and more), Nighthawk Rd. (classic sagebrush birding that gives you a shot at Sage Thrasher), and Anarchist Mountain (good birding and butterflying, including the ever-so-slight chance at White-headed Woodpecker... but that's daring to dream).
If you want a secondary location to consider, you could base yourself at Okanagan Falls and you'd be within striking distance of Vaseux Lake (Veery, Grey Catbird, nesting grebes and Bullock's Orioles) and the cliffs on the opposite side of the highway (Canyon Wren, Rock Wren, Lewis' Woodpecker, White-throated Swifts), Shuttleworth Creek Rd. (I believe I had Hoary, Western, and Moss' Elfins up there in one day, and it's good for woodpeckers, Western Bluebirds and more), River Rd. (Yellow-breasted Chat, Black-chinned Hummingbird, Grey Catbird), White Lake (Sage Thrasher and sagebrush country sparrows like Brewer's and Clay-coloured... I've seen Coral Hairstreak there, as well), and then Camp McKinney Rd. (Grey Flycatcher... I found a Western Skink there once) is a little further out.
If you really want to ensure you pick up loads of birds, you could consider enlisting the services of Chris Charlesworth of Avocet Tours:
http://www.avocettours.ca/. If he is not available, one of his faithful leaders can certainly put you onto loads of species. I have never met Chris, but I know he has a great track record and can probably help you pick up desired birds of the night, too, including Common Poorwill and Flammulated Owl. Something to consider if it's in your budget.
Good luck,
Jeremy Gatten
Saanichton, B.C.