I think most of them can be completed after the event ends provided you have purchased the items beforehand (the Easter Egg, Spring Robe, Spring Flowers, Tuxedo Shirt & Pants, etc).
You won't be able to gather any more chocolates, so make sure you buy everything you need and eat the 100 chocolates for the achievement.
Hard-boiled in Un'Goro HAS to be done before the event ends, though