I worked here in Winter 2016. I was assigned a team that had a lot of projects in progress. There is pretty good mentorship from people in your immediate team, but since the company is expanding very rapidly, it may not be as simple to grab a hold of people to ask questions. Work-life balance varies by team, with some people working pretty long hours, but as an intern, you aren't expected to work long hours.
The work you do at Snapchat will almost always be meaningful and be put to use. Those working on mobile will most definitely be able to point to something important or relevant in the app by the end of the internship.
There are many free food options around and a solid chef in the cafeteria for lunches and dinners. Although there's no on-site gym, you can get a subsidized gym membership at the one and only GOLD'S GYM.