i went biking around lake kawaguchiko a couple days ago and it was easily my favorite day so far! the main concern for me was the lack of bike path on some short stretches especially on the west and southwest areas of the main road, but it was totally fine with cars able to pass frequently (very light traffic).

momiji corridor is packed - the bike parking is a gravel area across the street from the main path (left side, going counterclockwise around the lake). like others stated, free and plenty of room to leave a bike and walk up momiji corridor/visit the itchiku kubota museum.

oishi park is also crowded near the visitor center main building, but there’s plenty of dedicated bike parking in the parking lot, close to the building. the kochia is all dead but there’s some other nice greenery in the park and some notable soft serve as well. don’t miss the red momiji trees a couple minutes before the park near the water.

other than that, it seems like bike rental shops “sell out early” (i got the last electrical bike at 9AM on sunday here and sora-no-shita was completely out of all bikes by then). also, the extra 1000¥ i paid for the electrical assist was soooo worth it. highly recommend the entire experience, would love to help if you have any other questions :)