I say if your dad and your grandparents are coming along, JERSEY BOYS, CURTAINS, and YOUNG FRANKENSTEIN would all be great options. I've seen 2 of those 3 shows, loved them both, and I'm sure YOUNG FRANKENSTEIN will be great when I see it in a few weeks.
Other than that, THE COLOR PURPLE is good, don't know if you want to see it for Fantasia or not, but she leaves in January.
But my overall reccomendation would be JERSEY BOYS. :)
My 2007/2008 Season:
Grey Gardens (7/5)
110 in the Shade (7/6)
Mary Poppins (7/7)
Xanadu (7/7)
Deuce (7/8)
Spamalot (7/8)
Jersey Boys (8/25)
The Year of Magical Thinking (8/25)
Mauritius (11/2)
Young Frankenstein (11/3)
Rock 'N' Roll (11/3)
Pygmalion (11/4)
Mauritius (11/10)
Mauritius (11/21) Mauritius (11/21)
Sunday in the Park with George (3/6)
South Pacific (3/7)
Gypsy (3/8)
Cat on a Hot Tin Roof (3/9)