Some are simple to find such as Riddler Trophies, patient interviews, etc.. (playstationlifestyle.net)
Again, these will be very handy for combat and investigation purposes, especially the larger list of Riddler challenges which adds a considerable amount of extra gameplay for the completionists. (dailyjoystickpodcast.com)
If side missions aren't your thing, or you've just completed them all, then you can amuse yourself by hunting down the ludicrous amount of Riddler trophies hiding around the map. (wolfsgamingblog.com)