If you go next week, the Flathead River will still be running pretty high, I'm thinking, so the fishing probably wouldn't be great.
Riding the ski lift to the top of Big Mountain (in Whitefish) is fun in summerAlthough I imagine that they still have quite a bit of snow this early in May.
Flathead Lake is great.
I always enjoyed going up and seeing Hungry Horse Dam, but once you get there and look off the edge, there's not much else to do.
You could also go by and see the old Cougarmeister childhood homes in Kalispell (it was a great place to grow up, and 40 years later I still sometimes wish I lived there).