First, thanks for your service in Afghanistan, be safe over there my friend.
Now about where to go. I'm born and bred in the Southern Highlands, and that's about the best time of year to be in my home hills, so I'm a bit biased. However, you mentioned GCNP. If you've never been to the Grand, and especially INTO the canyon, that's the obvious choice to me. Planning trips from the South Rim is easy as the Tanto trail will allow you to link trailheads to create as long a trip as you want. Maybe a loop from New Hance to South Bass? That's about 100 miles. You could use Grandview TH to shorten it, Boucher or Hermit to shorten it further. If you have a week, the shorter loops would allow some exploring on the other side of the river, maybe a trip across Utah Flats to Phantom Creek, or into the Clear Creek drainage, both are spectacular. There's just so many good options in the Grand.
Good luck planning for a great homecoming my friend!