The only "clue" in Manny's room is the picture of him and his blanket, and that is not especially helpful in the quest. In the house, the two useful clues are the Address Book and the Journal Page. But even these are only marginal suggestions, and neither is required to complete the quest.
The only important aspect of Manny's room is that exiting the window leads you to his tracks in the snow, leading across the roof. You do not actually have to leave that way.
Mannys room
Mostly the clues are in Greg's house.(Check Manny's room for the most important clue!)
It is in the school.
You can't open the door. It has no use for completing Wimpy Wonderland
in the laundry room
In rodrick's room
You turn off the elecricity in his room from the laundry room
in rodricks room
You go into rodrick's room.
go in Rodrick's room
get the dog bowl out of rodricks room
you turn off the power in the laundry room.