Select rides are open during the event. I would recommend doing all the HHN activites first though. They're time consuming if you don't have Express, and rides can be done any other time.
As said earlier, do ALL of the HHN attractions first as those are only open during HHN and won't be back the following year... Make sure to see the Bill & Ted Excellent Halloween Adventure show... Make sure you do all 7 Houses... and remember to have fun!