I was there for 4 weeks last feb and tried to do as much as possible, I'd say the guided walk on Fox Glacier was the best thing I did, just an amazing environment to be in. If I was going back I'd definitely step it up to the ice climbing option, saw others doing this and it looked great.
If you're in to tramping I'd say read up and try to find less well known options, as the DOC great walks, while beautiful, are too full of people and too sanitised in terms of challenge.
Most importantly, stop at every bakery you see, they are all brilliant!