Bachsee (or Bachalpsee) is a beautiful alpine lake at 2,266 meters above sea level. It can easily be reached from the cable car station at First. The hike is about 3.2 kilometers each way, and it is mostly flat. There are a few long but shallow hills. Though the station at First and the hike to the lake can be crowded, the scenery is incredible and worth enduring the crowds.
To reach the start of the hike, you have two options. You can hike from Grindelwald to First, which is about 8 kilometers and 1,200 meters of elevation gain. It's a strenuous but pretty hike. Alternatively, you can take the cable car from Grindelwald to First, which takes only a few minutes. Take a look at the timetable and information here. Note that you cannot drive to the trailhead, so you will have to do a bit of planning.
At First, you should check out the First Cliff Walk, which takes only about 10 minutes and offers great views. The path leads along metal grates and a bridge suspended on the side of the cliff! If you are afraid of heights, you may want to avoid it by taking the hiking path instead.
Bachsee is easy to reach - from the cable car station, simply follow the yellow hiking signs toward Bachsee/Bachalpsee. The hike is on a gravel path and is easily mountain bikeable. There are great views along the whole walk. There is a spot where the path splits in two - one path for mountain bikers, and one for hikers. Follow the yellow sign to the hiking path; however, they both lead to the same place. When you reach Bachsee, you can walk between the two halves of the lake. You can also turn right and walk up the hill toward Faulhorn and Gassenboden, which provides a great viewpoint of the lake. When you leave Bachsee, you can follow the same trail back.
Fishing is allowed at Bachsee, but only with a proper permit. Camping at the lake is also popular. This hike is great for families on a day trip as well - the lake would be an excellent spot for a picnic lunch!