Rich,
I'm new to the board and I'm not sure what I can add a huge amount to what bethbsting covered in her thorough review (great by the way), but there are a couple of things.
First off I went with a non-diver as well and (luckily for me) she was perfectly content to sit on the beach which I did my morning dives. I agree with Beth and don't use Dive Palancar. I would recommend using Dive Paradise. They will actually pick you up down the beach at the Allegro pier (a ten minute walk) if you ask. I noticed last year that the Grand had started a pier so it may be servicable by now for them to pick you up there. Ask for the small boat(6 divers) with DP and you won't regret it! As for your spouse even if she doesn't dive the snorkeling off the Grand beach was kind of nice. Nothing major mind you but I always found myself out there snorkeling at least once a day
Like Beth said the food was so, so but the rooms were beautiful and and the service was good overall. Even though we drank plenty of frozen drinks there and didn't have a problem (GI-wise) make sure to bring antibiotics with you in case you pick up a bug.
Not too many activites really at the Grand but that was ok with us. We went into town a couple of times for dinner, etc, but beware it's about $15 US (one way) for a taxi into town.
Clothing wise I wore my trunks almost 24/7 (except during dinner) because it was a might toasty (89-93 avg). I also agree with Beth and really don't think you'll need more than a couple changes of nice clothes (slacks, dress, dress-shirt, etc) because the rest of the time you'll want as cool a clothing as possible. One other item, I'm from Ohio but I can tell you if you don't get a lot of sun before you go bring a hat...LOL. I managed to cook the top of my head in a couple of days (ouch!).
Hope that helps you some! Have a great trip!