You can collect relics post game if you want, with the exception of one which you currently will not be able to access after reaching a certain point in the game (but there's a fix for that coming soon).
You can do all the hunting post game as well, but you won't be doing yourself any favours by postponing it.
There are a couple achievements that must be done before the end of the game, and that info is readily available in both achievement guides.
Originally Posted by Shabba
The mirror is staring right at the post which reads you are............. So hilarious that english makes them their posting.... Well done cheevo for confirming that you....... You must be drunk to not realise that or just another incoherent fool..