When are you planning on going? It can get really hot there in the summer time but it's very beautiful! It's also beautiful when it snows..you can probably take a helicopter ride that takes you over it.
Probably March. We're getting married on March 8, and then spring break is a week later (we're both teachers). I know that there are several different parts of the Canyon to stay - we want to hike (obviously) and are not afraid of roughing it. Neither one of us are luxury spa/resort people.
I went with my mom for my birthday two years ago (May 22) and it was snowing. We only went for a day. I would say pack for warm and cold weather. It was pretty hot during the day, but the evening got cold FAST. And like I said, the next day it was snowing in the morning. Since we were only there for a day, I don't really have any tips for you about stuff to do.
Havasupai falls is really roughing it though haha...FH and I are thinking about taking a backpacking trip next October though. Are you into whitewater rafting? I bet the rapids would be awesome at that time of year!