It depends what your walls are made of. Blu-tack, white-tack and the like can leave big greasy marks on walls if you're unlucky, and that's the kind of thing that they check for. Some people say toothpaste works but I've never tried that, and I can't see it working myself ... You can buy poster hangers but I think you need some where to hang those off, like a picture rail. Or, the other solution is to stick your posters to somewhere where it won't matter, like wardrobes and the backs of doors. It's likely too that you'll have some kind of pinboard in your room. Modern halls especially often have massive ones.
I doubt you'll be able to hang a clock though, you definitely won't be allowed to hammer in nails. If you've got a pinboard you could put a strong, long pin in that and see if that'll take the weight, or you could just hope that someone else has put a nail in before you lol. If there is one, just remember to mark that down on your inventory (if/when you get one) so that you're not blamed for it.