No, one night in Tofino is not worth the long drive over there--go for at least 3 nights if you go. I'd personally go to Tofino rather than Whistler but at this late date you may not be able to find any rooms there.
I prefer Orcas in the San Juans but only by a very slight margin--San Juan and Lopez are lovely too, although Lopez is a little quieter and has a smaller tourism infrastructure than the other two (which is not necessarily a bad thing--it just depends on what you're going to the islands for). At this point, it may well depend on where you can find a room.
What there is to do on Vancouver island depends on where you are (it's a huge, 300 mile long island). In Tofino, there's beach walking, hiking, sea kayaking, hot springs exploring, whale watching, fishing...plus just relaxing and taking in the scenery, because it's such a beautiful place.