Depending on the maturity of the kids, they may be able to stick to the mystery hunt. For my son's 7th birthday, we hosted a Super Hero/Villains party at home. It all had to be done inside since there was several feet of snow outside.
Here's a tip: Keep it simple and scour the internet for activity ideas! I debated over whether to purchase all the supplies to have them create their own hero/villain as part of the party-too complicated to keep the kids on task and too expensive. In the end everyone was encouraged to come dressed as a hero or villain. You could do the same with the Scooby gang and ask for them to RSVP with who they'll be attending as (ex: Little Joey Smith coming as Shaggy). This left me to focus on setting up activity stations-Pin the Spider on the spider web, Batman's Locator Hunt,Light Saber Battles, Pinata, Death Star Balloon Pop, etc. Here's a few tips to help keep the party madness to a minimum so you can enjoy the party you worked so hard to put together.
*A few weeks before the party-map out the activities including what, when, and where. This will help you get the maximum use of your space. You don't want to spend your time rushing to clean up after one activity to set up for the next.
*Set up your stations (or mystery hunt) before the guests arrive so you don't end up missing most of the party by constantly moving in too many directions.
*Have a pre-party activity set up to keep kids busy as all the guests arrive. We made hero/villain masks-you could have your bunch watch a Scooby Movie, color, or something like that.
*Have plenty of activities planned but set it up so if the flow of the party doesn't allow for them to complete all the activities they still solve the mystery. Kids get bored quickly so it's important to keep the party moving. BUT they also don't like moving on from an activity they're REALLY enjoying.
Good luck on your party! Hope it all goes well!