Lassen National Park [1] is dotted all over with tiny lakes—most are more like ponds or even puddles than true lakes. One of the most popular and most easily accessible of these is Summit Lake, which sits right along the main road.
The shining bright small lake attracts many campers to its two forest-shaded campgrounds [2]. There’s an easy walk around the lake that lets you see its waters and the plants that proliferate around it.
Or find one of the small trails down to the edge of water and eke out a spot on the miniscule beach with all the other visitors who come to escape the blinding heat. You can swim and fish in Summit Lake, and even take rafts and canoes out to paddle around. (No powered boats are permitted on any lake at Lassen.)
Links:
[1] http://www.moon.com/destinations/california/lake-tahoe-and-the-northern-sierra/lassen-volcanic-national-park
[2] http://www.moon.com/destinations/california/lake-tahoe-and-the-northern-sierra/lassen-volcanic-national-park/camping