Rimrock Lake is gorgeous!
This is an almost-exactly-100-mile ride that takes you on a loop of it, which means riding a lot of Highway 12 towards White Pass in the Cascades. The peak elevation by the lake is just under 3400 feet, making this a three-season ride in most years.
Highway 12 isn't the most quiet or best maintained stretch of road in the Pacific NW, but it's also far from the worst -- lots of nice views, moderate traffic, and generally adequate shoulder space. It's also graded very nicely on your way up towards the lake, with all of your climbing coming at a gradual 2% grade or so.
Your turn-off for Rimrock Lake comes at mile 36, giving you a full sixteen miles of glorious off-highway biking. It's all smoothly paved, and you'll see some campers, fishers, and boaters in the area on most days. There's an info control at mile 40, then another when you re-join Highway 12.
When you head back down the highway, be aware of one thing -- there's an unlit tunnel at mile 59, so pack a tail light even when you're guaranteed to finish well before sunset. There's no "bike warning" system, either, as you might find in other places (Highway 101 near Cannon Beach comes to mind); so, make yourself seen, ride straight and safely, and you'll be totally fine.
Aside from start/finish, the route has only a pair of info controls -- but there's several options for food/water stops along the route as needed.