I would go for St. Lucia to Grenada, one way.
Day 1: Rodney Bay to Soufriere, anchor at the base of Petit Piton, have dinner at Bennys Harmony Beach Restaurant
Day 2: Leave early make the boisterous crossing to St Vincent (gets rougher the last 1/4 of the way), anchor at Cummberland Bay take Dingy, taxi or bus to Wallibalou to clear in (officials arrive around 3-4 pm) have dinner at Mama Elmas, make arrangments when you arrive.
Day 3: Head south, lunch stop at Young Island Cut, snorkle on the opposite side of Young Island, make the passage to Bequia (can be a little rough)
Days 4 & 5 Bequia, grab a mooring off the Frangipani, walk around Port Elizabeth (get your fresh fruits and veg from stand around town rether than the Rasta Fruit Market, too expensive). In the morning listen for the conch shell horn, that means the fishermen are starting to come in (fresh tuna, dorado, wahoo for 8 EC per pound). Places to visit Jacks, upscale beach bar (saw Mick Jagger there once), Bequia Beach Hotel on Friendship Bay, nice walk across the island and lunch stop. Take a taxi tour(pickup truck with seats in back) around the island, should be about $30 US for a couple hours. Stock up on Captain Bligh Rum at Knghts Grocery, cheaper than on St. Vincent.
Day 6: Mystique if you want to see how the "other half" live, they charge you for 3 days ($80 US) even if you only stay for one. Or Canouan, take a mooring the holding there is poor and the anchorage can be rolly. Mind the dingy, the dock is notorious for damaging dingys or take a water taxi ashore.
Day 7: Tobago Cays, snorkle with the turtles, hike Barbadol (wait thats a shave cream isn't it, well it's spelled something like that) have a lobster dinner on the beach, Mr. Fabulous is the best, mention "the big man" from Skyelark and he will take care of you.
Day 8: Union Island, either a mooring at Cliffton or anchor at Chatham Bay (around the back side), you will have to take a taxi to Cliffton to clear out>
Day 9:Carriacou, they have mooring balls off Sandy island now or anchor at Tyrell Bay there is a customs office there now (I think).
Day 10: Grenada, spend a couple days exploring the island, lovely palce with some of the friendliest people in the Windwards
You could drop a day in Bequia to give you a little more time somewhere else.
Itinerary looks a little rushed, but we often do a 10 R/T Rodney Bay to Petit St. Vincent, trip report is in the archives. Enjoy your trip!!