Cam's proposed itenerary is an excellent suggestion, but I wonder about Canouan.
We've been there three times (twice to pick up and deposit guests), and didn't find much of interest compared to the other islands. I mean, it's a pretty island, as they go in the Grenadines, and we had a nice walk around, but really didn't find much else to do. And the swell was so bad in Charlestown Bay that we opted to head for Mayreau rather than spend the night there--we definitely wouldn't have been able to sleep, we were rolling so badly.
South Glossy Bay is pretty nice, but I'd be willing to bet you a Carib that the little French hotel/restaurant/bar there is out of business by now. They were barely hanging in there when we were there two years ago. The bartender told me that the Canouan government had agreed to limit hotel room availability on the island to promote the Raffles development on the northern end. The locals were incensed about it.
Here's Chris Doyle's summary of Canouan: http://www.doyleguides.com/canouan2.htm
Visiting Mustique is an "experience", and well worth it to some. My wife, Lynne and her visiting friend, enjoyed seeing it. I went along to carry the credit cards. But you should know that, last time I checked, they were charging $25 US per night for moorings, with a three night minimum, i.e., $75 US. We visited Mustique on a day sail on the Caribbean schooner, The Friendship Rose
. She leaves Admiralty Harbour on Bequia in the morning, and you're back by 4:30 pm or so. They do a fantastic job of wining and dining their guests while on board, and you get to see a genuine piece of Bequia's sailing history up close.