This is a charming small lake which sits at 6,895 ft. in a small depression on a ridge, with a few excellent camp-sites. There are several great views of the surrounding countryside both during the hike and at points around the lake. It's a good destination either for a dayhike or for an overnight trip. During my trips to Penner, usually in the fall, I've rarely seen anyone else at the lake.
From the Grouse Ridge trailhead, according to my topo software, this is about a 8 mi. hike, round trip. There's a net elevation change of just under 2,000 ft. The trail is well-maintained.
Take I-80 east, about 85 miles from Sacramento, to Highway 20, and turn north toward Grass Valley. 5 or 6 miles along Hwy. 20, turn east on the Bowman Lake Road, for 6 miles. Just past the PG&E camp, turn south (right) on the Grouse Ridge Lookout road. The road becomes dirt & gravel. While 4WD isn't required during the summer, it's good to have a vehicle with relatively high clearance...lots of potholes, rocks, bumps. In spring/early summer or after an early snowfall, don't risk it in anything but 4WD. It will probably take at least 25 min. to drive the last 6 miles on the dirt road. Keep left, beyond the Grouse Ridge Campground, to the trailhead parking lot. While one can also park and hike from Carr Lake, further along the Bowman Lake Road, it adds another 1 or 2 rather unattractive miles round-trip to the hike. Carr Lake is also often crowded. I recommend using the Grouse Ridge parking lot as your trailhead.
Penner is one of the nicest small alpine lakes I've seen. There are numerous great spots for swimming and picnicking. The trail around the lake itself is somewhat challenging, with opportunities for rock scrambling on the western side. Because the lake sits high, in a small depression on the western side of a ridge, there are good views to the west. The lake is clean and cold, and fishermen with whom I've spoken have had good luck there. One fellow caught a 14" trout on the 1997 Labor Day weekend.
Carry 1 to 2 quarts of water, depending on the temperature. There's often running water in streams along the trail, but there are also summer-grazing cows in the area. It's probably wise to treat even the running water. Check the area weather before you leave home. There are often thunderstorms in the summer, and occasionally late or early snowfalls. Bring extra clothing to the trailhead and decide at that point what you want carry on your hike.
Here's a rough map of the trail from the Grouse Ridge trailhead to Penner Lake (83 KB). The trail is an approximation only.