In order to collect ALL the stars on the levels that have the Niko-face wooden platform Niko will need to have a specific costume and use his special ability at certain times. The wooden platform will indicate which costume Niko needs and what part of the map he will need to use his special ability in order to progress.
Extra Coins
Although there are more than enough coins in the game to purchase ALL the available items you can always earn extra coins by replaying levels.
Muddy Areas
The areas that Claude and Terrance can travel underground are marked by muddy areas which usually appear on either side of a hedge wall. There are some hedges these characters can travel through, to determine if a hedge is navigable you will need to draw a line into one to test. If they can travel through it, your line will continue. If not, it will be diverted to one side.
New Events and Levels
Keep an eye out for green exclamation points when you are on the island map or a specific region's map as these indicate either new events or levels that you have not yet attempted.
Timers
Remember that the timers do not start until you walk or activate an ability so make sure before you start you scout the level and prepare your attack.