These are all the current levels with a book, rainbow, or secret bonus that could be faster to take that I am currently aware of; this will be updated as new ones are found

sand land 1 in a block to the top left of the 2nd enemy from your starting location 4 - between pits of water 8 book at bottom right of level 5 - to right of square of 4 ! boxes

wet land 1 - get powerup that shoots down and use it at beginning of level to get a bit, or go to top of level and get a rainbow 3 - left block of rectangle at beginning of level 4 - to right of the middle of level 8 - at beginning of level 6 - at top of level to the left or zip down at beginning 7 - near topright of level, can also zip down using a big platform at the bottomleft of the level

plastic land 8 - at beginning of level near where dog is at top 5 - probably early half of level

sand land 2 1 - toward beginning of level to left after rows of snakes 4 - on right side of pillar, gotta be fast and will want blue potion 5 - book at end of level 6 - toward end of level 7 - at bottom left, can go through wall

wet land 2
1 - at middleright block at the beginning 2 - book at end of level 3: right before breaking blocks section with sharks, to the right 4 - at 2nd screenwrap to the right at that block there 6 - in between 2 rows of disappearing ! blocks at the block at the middleleft 8 book at topleft of level

plastic land 2 3 - at a block to right at bottom of level 4 left block at top of level 5 - in one of the blocks above the row of dissappearing floor

future land 2 1 in block after first ? section in bottom block below spring 2 - in ! blocks after conveyor near start of level 5 can hold left and b at the beginning of the level and you will clip in the ! blocks and fall to a secret bonus 6 book in alcove at middletop right, rainbow around that area as well 7 can bounce into the ! blocks and past them and fall to a secret bonus