Oh we stayed at the grand st. lucian for our honeymoon too!!! It was beautiful in St. Lucia, still one of my all time favorite places. We took a taxi with another couple we met into town to the market place, which was pretty interesting.
The drive from the airport itself is very interesting! On the way back our driver stopped a few places so we could take some really great pictures! We pretty much stayed at the resort, but we did take the sandals shuttle and check out the Regency one day. I would suggest just going there and looking around or spending the day at the beach, its really different from the grand.
If you have any questions or would like to see pics, PM me!!
I hope you have an AMAZING time!!!