There are four secret rooms with items in. If you walk around a bit you can go through various buildings and you'll find rooms with puzzles in. (Two of the rooms are only accessible from Union Cave.)
You can solve those puzzles to make a whole in the floor and make unown start appearing. But at the back of those rooms are messages on the wall. One says "LIGHT", for example, so you need to use Flash in front of the wall to open the secret room.
I'll leave you to figure out the other secrets, they're not too difficult.