We like to go here and rent a cabin. The lake is so pretty in the summer. You can walk almost half way across it which is fun. The cabins are amazing and cozy. Make you wish you could live like this all year round. Very kid friendly. Mine especially liked the general store.
Lake of the Woods is a gorgeous mountain lake about 1.5 hours from Medford OR, or 1 hour from Klamath Falls, OR. Check the website for cabin rentals (reserve early!) or just go for the day! There's a great swimming beach with many semi-private coves (and neaby parking), a marina, and even several dining options including a pizza parlor.
There is an entry fee of $4 per vehicle for day use.