The campgrounds off Highway 35 can be quite pleasant during off-peak times, and crowded and noisy during summer weekends.
Finley Point State Recreation Area (406/887-2715, http://fwp.mt.gov [1], May–Sept., $15–18) is four miles off Highway 35 on cherry-tree-studded Finley Point.
The Yellow Bay State Recreation Area (406/752-5501, May–Sept., $15) and campground is right next to the University of Montana’s Biological Station [2].
Links:
[1] http://fwp.mt.gov
[2] http://www.moon.com/destinations/montana/missoula-and-northwestern-montana/flathead-lake/east-side-flathead-lake