I just returned from St. Lucia and St. Vincent and the Grenadines Monday ... I love that my land legs aren't quite back yet.
I agree with all of the above suggestions and would include Union Island (Chatham Bay ... have BBQ with Shark Attack).
One thing to consider about Mustique is that they are tightening the screws on cruisers and what you can do on island. You can pay to take a taxi ride but you can't hang out at the nice beaches. We skipped it this trip and didn't miss it at all.
In Canouan look for John who runs half of the mooring
balls (his have orange on them I think). He provides exceptional service
and has a water
raft he can bring out to top off your fresh water
I cleared in at Canouan (night sailed down from St. Lucia) and cleared out of St. Vincent at Wallilabou on the way back to St. Lucia. Are you doing a one way (south) or do you have to get back to St. Lucia? If you have to get back north be prepared for a bouncy sail between the islands with the wind
on the nose.
We also enjoy Young's Cut on the mainland (St. V) and our kids
love climbing the islet Fort Duvernette.
You can't really go wrong. On the mainland (St. V and St. Lucia) make sure you stash your valuables (electronics, passports, cash) or keep them with you and lock up tight. Theft is on the rise particularly Soufriere, St. Lucia. If you are smart about it no problems.
Fair winds and have a great time.