Katmai is a magnificent and amazing place. I think that a day trip really doesn't allow you to see enough- often daytrippers are restricted to the first, less active, viewing platform so that they won't be trapped by a "bear jam." The trip to the Valley of 10,000 Smokes, site of the 20th century's largest volcanic eruption, is well worth it (as is the backcountry hiking through the ash).

The NPS has a good campground at Katmai - only 8 bucks a night. Just be sure to reserve online well ahead of time.