If you want to stay at the resort, then the suggestions given above are good. Since you are staying at the Grand, you may want to look into whether they still do the hotel-guest early morning exclusive entry at the Build a Bear that I have heard about if that is of interest to your group. I have also read about a hotel tour, so I guess that may be a way to fill time. Meals, shopping, swimming, a movie at the movie theater, wandering the grounds of the three Disney hotels are all ways to fill time.
If you are looking to get away from the resort for your day six, there are lots of possibilities. Fairly close by is the Discovery Cube in Santa Ana, a great hands-on science museum that my boys loved. I believe there is an ART route that goes there, so car rental or taxi wouldn't even be necessary if you don't mind being on a bus timetable. Knott's Berry Farm is only about 8 miles from the resort. While it's no Disney park, it does have it's own quirky charm. Again, there is an ART route to Knotts although I don't think that's an especially convenient mode of transportation for that outing.
If you want to go further afield, car rental is probably best. Mary Jo has done a bunch of great Day 6 segments. They are referenced again in this recent thread:
Day Six Ideas for 2016?