Here's a hint: All the collectibles EXCEPT ONE can be seen from regular, non-secret areas in the levels. The hidden one is in the second part of the two sewer levels, as a reward for discovering something else in a hidden area...
I jsut seriously hope that the hidden stuff isnt outside "the safety of Hova" sicne getting over those spikes is hard enough (maybe like every 5th try or so works. but getting up on those ledges, I jsut cant do it :-(
