You'll love it... all the places mentioned are great, Cassell falls and lake are our personal favourites, and it's possible to overnight there but you need to take some care anchoring, it's a challenge. Most do the day trip and return to Squirrel. But there's something to be said about having the lake to yourself late in the evening or early in the am.
Check out the west side of Cortez Island too, if you have time, much less traffic - Von Donop inlet is pretty and secluded, Robertson Lake is clean and quiet.
We generally avoid Prideaux Haven and much of Desolation Sound Marine Park because of crowds, but other places for options include:
Tenedos Bay and its trail to Unwin Lake
Roscoe, as mentioned, and Black Lake
The head of Pendrell Sound for warm water
Gorge Harbour for it's interesting entrance complete with Petroglyphs
The north side of Savary Island as a lunch spot on your way out of Comox
Copeland Islands for your first overnight stop.. try the west side rather than the popular bay on Thulin Pass.. it's a major thoroughfare and has lots of traffic wash and current as well.
In 6 days you'll be pressed to do them all, though they are not any great distance apart - you'll pick one and probably just want to stay there!!
But must dos would include the Squirrel lagoon and Cassell Lake and Falls.
Can't really go wrong, hope the weather does well by you.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)