River Lodge

This hostel was a great find - a quiet, lovely location on the lake, with clean rooms and bathrooms and a kitchen. Rooms were on the small side and a tight squeeze at times when both bunks were full, but it was perfectly adequate. There was one sheet provided (I'm not sure if they do blankets in the winter?), but I felt more comfortable having brought my own sleeping bag.
It's within about 5-10 minutes' walking distance from the centre of Interlaken and right across the lake from the train station Interlaken Ost (although it takes about 10 minutes to walk as the closest bridge is a bit further along).
It's both a campsite and a hostel - it had a school camp sort of feel.
I chose this place over the other main hostels as it is the closest to the train station, and, travelling alone, I was a bit intimidated by the idea of the larger hostels that have a reputation for loud parties late into the night. At River Lodge there were still other backpackers to chat to and swap stories with, but also families and school groups which gave the place a quieter atmosphere.
The staff were very helpful in booking activities for me and the coffee that they sell at reception was surprisingly good!
There were other travelers there who had some problems with contacting the staff (as reception closed at 6pm and didn't open until 8am). This was resolved by calling the staff on the phone provided outside reception. I met another girl who arrived late at night. She had already booked and there was a letter for her on the door of reception with the pin number to the safety deposit box to get her key from, but the letter had already been opened and the key gone from the box. Luckily she didn't need the key as she was staying in a dorm where others were already inside. All these problems were resolvable, but, if you can, it's best to make travel plans so that you check in and out during reception hours.
It felt otherwise safe here. They provide lockable boxes for each person in shared rooms so you can put away your valuables. They also claim to have luggage storage for during the day after checking out, but every time I asked about this, I was told just to leave my bags in the kitchen. I did this and had no problems (but also note that there are lockers available at the train stations for those who aren't so trusting).