Mark Z
Google
The marimo are interesting and adorably round, like Tribbles from Star Trek.
In mid-October, the colors are beautifully intense, the weather is enjoyably cool, and the views are amazing.
The boat tour was 2400 yen; quite decent considering we not only went around the lake, but also in the observation facility in the middle of the lake.
The marimo ice cream was good, though I thought I tasted a faint green tea flavor. I recommend the Lake Observation area; it's a short hike and the view is great.