When you are outside, look for small patches of grass on or near pathways. If 
they flash when you walk by them with your wand out, try casting Accio on them. 
A hidden Fred and George Parcel may appear. You can find these most commonly in 
the Stone Circle and behind Hagrid's Hut (including the pathway into the woods).
Discoveries in Room Of Rewards 
Use Wingardium Leviosa on the feather duster in the Trophy Room. Hold it over 
the two dusty trophies next to it, and it will clean them. Then, examine each 
trophy. Those four Discoveries will register a 100% on the Marauder's Map.