Before you can do anything in Charlie Brown's backyard (left of Main Street houses), you have to go right to the Pumpkin Patch and perform two tasks: weigh a pumpkin for Lucy, then roll it back to her house safely.
Take Linus's trick-or-treat bag and give it to Pigpen, then give the sucker to Linus, then pick up the pen. Now you can go left to where Charlie Brown and Snoopy are gathering leaves.
He is with Snoopy in his backyard, but only after you have gotten a pumpkin for Lucy, a bag for Pigpen, and a lemon sucker for Linus.
There is no pigeon on great pumpkin island.
The pumpkin patch is to the right on Great Pumpkin Island. It was the only area shown in the island preview.
There are no buildings or upper rooms on Great Pumpkin Island.
To get to Charlie Brown's backyard, go left past the Flying Ace cafe. You need to complete the pumpkin patch first, then get the pen and invitation, before you can do the Snoopy and Charlie Brown scenes. (see related question)
The pumpkin patch is both the first and last point on the island quest. You go there initially to see Linus, and return a pumpkin to Linus and Lucy's house. From there you swap Linus's bag for a lemon sucker, and swap that for an invitation to the party, before going left into Charlie Brown's backyard.
The name of the island is Great Pumpkin Island. See the related question below.
you wait for the great pumpkin!
At the very end, there is a mysterious figure in the pumpkin patch!
when you go to the snoopy food court [where you first enter] go to the left and that's Charlie Browns back yard
There is no potato farm on Great Pumpkin Island.
Great Pumpkin Island