I figured you were on a smaller ship when you mentioned Bequia, the big ones can't get in there. The Sea Cloud stopped in Carriacou yesterday, the first cruise ship to ever stop on the island, they're due to make 5 more stops this season.
On St. Lucia you'll probably be anchored off Pigeon Point rather than the harbor in Castries. Some of the ships, especially the smaller ones, offer disembarkation in the north and pickup in Soufriere; if yours does that would be a way to save time by not having to drive back up north if you decide to check out the nature sites.
Grenada is picking up with cruise traffic, last week the chairs were 4-5 rows deep at the spice market, I've never seen anything like that in 10 years of visiting. We're considering skipping Grenada all together in the future and just heading straight for Carriacou, and Grenada has always been our favorite island. St. Lucia is a very popular cruise stop and it's not unusual to see 2 or even 3 ships in port.
You can find the schedules for both islands online which might help you decide what to do.