C.P.
I love planning kids' parties! That being said, I have found that kids this age really don't want a lot of structured activities--they'd rather just play together. The "games" they think are the most fun are the ones where everyone participates at the same time, like a treasure hunt, a dance party, a "snowball fight" using balls of foam, that sort of thing.
Personally, I would go to your friend's house. Kids may not have Halloween costumes yet, so I probably wouldn't do that. The pirate themes suggested by others would be fun. You could also do a superhero party. Capes are easy to make and are great party favors too. Just buy some red fleece (or whatever color you want), cut into rectangles (about 15" x 30"). Then a couple of inches from one end, cut out a circle for the child's head to go through (just try it out with your son for sizing). Then round the front corners out to make it look nice. For extra safety, you can snip open the front of the circle and sew or glue in velcro. As an activity, you could provide felt pieces and glue or foamies stickers and let them decorate their capes. Superhero cake decorations can be bought just about anywhere.
Other ideas: You could base your theme on your son's halloween costume and let him wear it for the party if he wants. A make-your-own taco bar is probably the easiest and least expensive way to do food. Put beans and meat in crock pots if you have them (or can borrow). Everything else in bowls for people to serve themselves.
If you want any other ideas, feel free to email me! My husband thinks I wanted another child just so I could plan more birthdays...