There are bound to be a lot of recommendations in earlier CF threads on that route
if you use the Search function.
- - But briefly, the hot spots to hit are Marigot and then the Pitons; skip all of the main island of St Vincent; then stops at Bequia and the Grenadines including Mayreau; Union Island; Petit St Vincent and Mopion Island; Carriacou (check in to Grenada
at Hillsborough) and Tyrrel Bay; The St Georges on Grenada and all of the southwest bays of Grenada (Prickly Bay; Mt Hartman; Hog Island; Clarkes Court Bay).
- - With only 10 days you will be really pressed to just get from St Lucia
to Grenada with only an overnight stop in most of the places.
- - Be sure to get Chris Doyle's Guide to the Windwards. It is invaluable.