Hi, Zion has plenty to do, i guess your fitness levels will have a lot to do with you filling in your 6 hours :)
Canyon Visitor Center is a good place to start. There is a Canyon shuttle service, that takes you through 8 locatons through the park, you can park your car in Springdale and use the shuttle stops. There is also plenty of hiking and Bird watching, Zion is home to 290 species of birds. Bird checklists are available at the visitor centers. California Condors, which were released in Vermillion Cliffs, Arizona, in the late 1990's, are increasingly being sighted in
Zion National Park you will find plenty of information on how to fill your day.
Hope this helps.