One of your first stops should be Hallenberg Visitors Centre. Here, you'll be able to orient yourself and get lots of useful information before you set out exploring.
You're spoiled for sightseeing choice when in Hallenberg. Hallenberg Visitor's Centre is a great place to begin here, but there are loads of other fine places to check out. Outdoor Theatre Hallenberg, and Hallenberg City Hall are close to the tourist information center and and will cap off your day nicely.