Okanagan Mountain Provincial Park is my pick.
IIRC, it's easist to get to by boat, but there are some dirt roads that will take you there from Kelowna/Vernon/Penticton.
It's like a backcountry Rockies campground in the Okanagan. Here's an info link I just dug up:
http://www.britishcolumbia.com/parks/?id=147
Edit- Looks like the roads only take you to a parking lot, and you hike in. I don't think it's all that far though- we arrived via boat, and although I remember checking out most of the park, I don't remember exactly how far away the tent sites were.
One more edit- If you wanna venture a little further north, past Kamloops, the Myrtle Lake area is breathtaking.