We've probably spent over two months in the Grenadines and love it. However, if you're starting from
St Lucia, it's a stretch unless you're doing more than a week's
charter.
I don't know where your charter begins, but Kesey is right--go from Soufriere (the Pitons) directly to Bequia, about 12 hours sailing and motor-sailing. We have made the decision not to stop in St Vincent.
His suggestions for destinations in the Grenadines are great, and I would add Clifton Harbour on Union Island. The problem is, you have to clear out of
St Lucia and clear in to SVG, and then clear out of SVG and clear in to St Lucia when you come. That takes time and you can only do it when the customs/immigration offices are open. Really, if you want to sail in the Grenadines, you should charter out of Canouan or Blue
Lagoon on St Vincent to maximize your time there.
On St Lucia, you have Rodney Bay, Marigot Bay and Soufriere/Pitons. You could spend two or three days at Soufriere and take a
water taxi around to Anse Castelet, and also over to the
hotel between the two Pitons. Both have good restaurants for lunch, good beaches and good snorkeling. If you pick up a
mooring at the Bat Cave moorings, you have good snorkeling right off your
boat, and a great restaurant at the Hummingbird.
If you want to get out of St Lucia, you might want to consider heading up to
Martinique. Spend some time in St Anne--lovely little town. And you could slip around to Grande Anse d'Arlet.
Martinique is closer to St Lucia than the Grenadines, and the
food is better
.