Having not been aged 17 for nearly 19 years myself now, I can't speak for what people your age like to do, but I can certainly recommend a few things that myself and my peer group like to get up to.
Eating: There are a huge number of awesome restaurants and the like in Brighton. I'd definitely make it a point to try out as many as possible whilst you're here. Don't forget, it's important to stuff yourself just to the point where you think you might die, but are never actually granted that mercy. As Louis CK says: 'The meal isn't over when I'm full. The meal is over when I hate myself.'
Sleeping: Sleeping is great fun. We do it as much as we can. Can't recommend it enough.
Entertainment​: Netflix, Amazon Video and the cinema are all great ways to temporarily escape from the mundanity of the daily grind as a fully functioning adult.
Funeral Parlours: Planning what happens to you after your final sweet, sweet release from the grip of this mortal plane can be a fun and cheap hobby to engage in, alone or with a partner.
Drugs: Marijuana is an awesome way of helping you cope with the fact that every day spent working on this Earth is another day closer to death and not doing something that actually stimulates you mentally.
Like I say, I can't speak for your generation but these are all things my little circle of friends like to get up to. (4 people isn't technically a circle I know, but I guess it depends how you stand).
Oh, and there's fidget spinners fucking everywhere it would seem.