Flathead Lake is huge, and gorgeous. It's the biggest lake in Montana and certainly one worth spending some time on. The water is surprisingly warm, and the views are amazing. If you are passing through the area, find a place to rent some paddleboards and hit the water around golden hour in the evening. 

A great place to explore on the lake is the northern part. It's the closest area to Kalispell and to nearby Glacier National Park. After spending time there, an evening paddle is a great way to relax. For a launch point, I recommend the North Flathead Yacht Club boat ramp. Paddling on this northwest side lends the best possible view of the Mission Mountains to the east. Also, from here there's an abandoned lumber mill about .25 miles away that you can carefully explore. There's access to walk on the docks and the inside of the building, which still has some machinery still surrounding it. Also, the mill used to have boat docks that have been all but washed away, except the wooden support beams remaining. Bring hammocks and string them up in between these for a sweet hang spot. 

For a bonus, pack a picnic dinner and eat it over the water!