Each level you play (trials excluded) throughout the single player adventure mode gives you the opportunity to earn up to three optional objectives. You can look at these objectives at the level select and by pressing
To complete 30 of the 180 is relatively easy; I got this playing my way through the game without putting any real emphasis to completing the optional objectives.
Tip: Know that every level has "clear all pegs" as an optional objective, so if you find yourself in a situation where you can pretty easily entirely clear a level, it might not be a bad idea to do so.
Plus, when you do clear all the pegs, you'll bag the "Ultra Extreme Fever" achievement and/or the "Coolest Clear" achievement, depending on how you clear it.