If you have a few days to explore the Lassen National Forest area and are looking for water activities, spending sometime boating or swimming at Lake Almanor is a good option. However, if you have only one day or you are looking for hiking through pure nature then I suggest trying the nearby Lassen Volcanic National Park instead. Lake Almanor is a large man-made lake and is developed with houses and boats, jet skis, etc. The Lake Almanor Recreational Trail is a nice easy walk around the south side of the lake that is peaceful and pretty. At around 4500 feet it is cooler than the surrounding valleys, but still warmer than Lassen Volcanic Park (over 7000 feet). If you have your own boat there are several boat launches (also on the south side).