I biked the entire loop around Crater Lake on a beautiful sunny day in early September. (I drove up to the park, however.) It's a gorgeous but exhausting ride because it's mostly up and down with some long climbs. I read that in the 33 mile loop around the lake, the total amount of climbing adds up to about 3,800 feet. Parts of the road are away from the rim with no view of the lake, but it's still wonderful.
BTW, I was back in the national park in late September a few years ago, and it started to snow and the lake quickly ceased to be visible.