C.M.
I think having a party before trick or treat sounds fun. A good way to kick off the nite. I would definitely make it a costume party. That way they can go trick or treating straight from the party. Serve healthy, nutritious food since they are gonna get a bunch of candy right afterwards. Maybe have one or two games. They don't have long attention spans at this age so I wouldn't do too much organized activity yet.
However, if you don't like combining, could you just have her party the weekend before or after? Maybe do a fall themed party instead? Do a fall craft, like a leaf collage. Serve cider, maybe bob for apples, bury the kids in the leaves, etc. If you live where it is allowed, you could have a campfire and roast marshmallows and hot dogs.If it's before Halloween you could even carve pumpkins.
GL and have fun with the party! Your DD will have a blast no matter what you do, after all it's a party!!