they are excited!A scavenger hunt sounds cool! *writes idea down*.
We have done different games from year to year. Just do some searches and you can find lots of ideas. We always have music playing in the background outside. 2 firepits going, where everyone seems to gather. I have some books that have "friendly" campfire stories so we always have a story time. Everyone gathers around for this. It's when the s'mores are made and the families get to enjoy each others company as one big group. If the weather is less than desirable I always have some sort of Halloween movie playing inside. Our house is always decorated so the mood is set. 
There is some very good advice for you here. My first Halloween party I had had 3 people show up!!! That was it, I now have an average of 20 people show up and people are always asking me what's going to be new this year. I started out making all the food myself but that gets expensive quick. Now I ask half of my guests to bring a side dish and half of them to bring dessert (we had basically everyone bring dessert one year). The giving a prize for the spookiest food is a great idea that I will use next year.This will be our 8th Halloween party. They have always been family oriented and have always been successes and people look forward to it every year. I was already asked back in May if we are still having one. lol
We always have a costume contest. There are bag toss boards set up. Mine are Halloween themed so it works.
For food we provide a couple of the main items and beverages. We started asking for covered dishes, Halloween themed of course, and offer a prize for the spookiest dish.
Since this is your first Halloween party let me offer this, if things don't go exactly how you envision them don't let it discourage you from having future ones. We've seen so many first timers post that they were so disappointed on how things turned out that they don't have any more. If not all your guests show up, even if they rsvp, don't sweat it. It happens to us all. If something goes wrong or doesn't turn out like you wanted, guess what, it happens to us all. Have a guest that you realize was a mistake? Yep, we've all been there. Just don't invite them again. It's all a learning experience. Be excited, do what you can, take notes, and just enjoy it.
I always hope/plan for outdoor activities but we usually end up inside due to weather (that's a mixed blessing because as much as I want to do stuff outside its kinda nice that's it's been so cold, I can put all of the cold beverages out in the screened in porch and they stay cold and we don't have to make room in the fridge). Another tip is if you can find invites now (check places like party city) get them, fill out and address them. That way when your ready to send them out you have them all set to go and you can go back to party preparations.
No problem.
Anything to make the setting up/final stages for the party easier is good. I'm inviting some people from out of town so I'm planning on sending mine out early to mid September.
