It would be tough to get to the Cays in that amount of time. Could try something like this:
10th - Marigot
11th - Marigot to Soufriere, nice short morning sail to get familiar with the boat, nice anchorage near Petit Piton,then either dingy to town and walk to the Botanical Gardens or pre-arrange an afternoon dive, the diving is said to be good near the Pitons.
12th - Soufriere to Wallilabou, clear into St. Vincent, have dinner at the restaurant with the Pirates of the Caribbean set.
13th - Hike La Soufriere volcano,from the leeward side you get to see 3 different eco systems, the sea shore and rivers, the rain forest and the high peaks.
14th - Wallilabou to Bequia
15th - Scotter rental for a trip around the island or diving
16th Bequia to Soufriere, leave early it will be a long and trying sail
17th - Soufriere to Marigot, stop at the snorkling spot just north of Soufriere near one of the big resorts (can't remember the name of it just now, but Kim said it was the best snorkling she has ever done)
18th - Head back home and remember, you can't come back unless you go home first, kinda takes the sting out of it.