Went on one which was a photography session in Toronto with a professional photographer (with only 2 other "students". The host was very friendly, generous with his time, knowledgeable, and I learned a lot.
In cities, even in places you've lived your whole life, it's often really hard to find interesting activities. AirBnB experiences are presented in a beautiful UI from an already reliable web platform, and there's a lot to choose from. Makes a great gift idea too. Only thing is that they can be expensive.