Slocan Valley Route
A nice long day drive I enjoy quite a bit is the loop around the Slocan Valley area. This takes you through Castlegar, Slocan, Silverton, New Denver, Kaslo and Nelson. This area can be accessed from Kelowna either by highway 6 (Lumby/Cherryville) and the connecting ferry, or the route I personally take which is highway 33 through to highway 3. See the map below.
This is a long drive but offers great scenery and is an all around fun route.