Museum Puzzle
One of the first puzzling safes you will run into is in the museum portion of the house. Next to the door is a scrambled pattern that must be solved in order to get what's inside. The pattern is a dollar sign with dashes to either side. With this in mind, it becomes easy to move the pieces around to form the dollar sign. Once you have unlocked this safe, gather up the clues inside because they will be used later for more difficult puzzles.
Window Puzzle
Later on, as you search through the museum room, you will see a dirty window that is really a puzzle. You can draw monetary symbols on the window to open it. The problem is that you can get to this point without knowing the proper solution. The correct order of symbols is "€ ¥ ₤ $ ₤ $ € ¥ $ ₤ ¥ € ¥ € $ ₤." Once you have drawn the correct symbols in order, the window opens and outside there are more clues for later puzzles. Make sure to gather those up.
When All Else Fails
You will run into problems with some puzzles. Some puzzles also become impossible if you make a wrong move, but you're able to reset each puzzle. In order to reset a puzzle, press the escape key "ESC" to close the puzzle, then leave the room. When you return, it will be as though you had never started the puzzle. This is especially beneficial because any items you might have used in the puzzle return to your inventory.