After Snoopy gathers the leaves and Linus jumps into them, follow him as he mails his letter, and come back for his blanket. After he mails the letter, you can go left to where Charlie Brown and Lucy are using the football. Let Charlie Brown use the pen.
You give it to Charlie Brown, and he will sign Lucy's contract
Normally you get the pen when Linus gives you the invitation, because he has been writing a letter to the Great Pumpkin. You will then you go left of Main Street to have Snoopy gather leaves in the back yard. After the leaf scene and after Linus mails his letter, go left again and Lucy and Charlie Brown will be playing with the football. When Charlie says "Please find me a pen," click USE on the pen..
use the football
After you give him the pen, he is again tricked by Lucy. When they leave, pick up the football and take it left to Snoopy's doghouse. Use it to knock his flying helmet down from the tree.
You have to go to Linus (by the mailbox) and he will send his letter to the Great Pumpkin. If you have the Pen from his house, go left and play the football scene with Lucy and Charlie Brown. You can then use the football at Snoopy's doghouse to get the flying goggles and start the Red Baron scenes.
You can get a mask, a sucker, an invitation, a pen, a blanket, and a football. P.S. You have to use most of those for the game tho so you would only keep the mask and the invitation in the end.
You have to give Linus the yellow (lemon) sucker you get from Pigpen. Linus is going to wait for the Great Pumpkin instead. -- When you get the bag from Linus for delivering the pumpkin, take it outside and give it to Pigpen (on the sidewalk, right). He will give you a lemon-flavored sucker, which you can give to Linus. He will give you his invitation, which he wasn't going to use anyway. Pick up his pen from the desk to use later.
Take the pen that Linus was using in his house, and let Charlie Brown use it in the football scene. When Lucy and Charlie Brown leave, take the football to Snoopy's doghouse and use it to knock down the mask.
You just click on the leaf pile when you are hunting for Linus's blanket.
That is not a feature of the island. You can get a costume change using the costumizer by pressing the 3 keyboard keys Ctrl+Shift+P all at the same time. You remove it the same way. You cannot use this or any other costume for the halloween party on Great Pumpkin Island. You have to get the mask from the scarecrow in Snoopy's adventure.
As soon as you get the bag from Linus, take it outside and give it to Pigpen, who will give you a lemon-flavored sucker. Bring this back to Linus and he will give you his invitation to the Halloween Party (he'll be in the pumpkin patch instead). Pick up his pen at the same time.
Go left to Snoopy's doghouse and use the football. It will knock Snoopy's helmet out of the tree.