No lighthouses within walking distance, but there is one on George's Island across from where the ship docks:
Unfortunately, it is not open to the public.
Probably the easiest lighthouse tour is Peggy's Cove; there are ship tours available (Carnival's "Lobster and Lighthouses" claims three lighthouses, including Peggy's Cove). We went with Your Cab, a private tour company (yourcab.ca) to drive out to Peggy's Cove. We managed to get a rather foggy day, so our tour of the coast wasn't the greatest:
However, Jonathan was a great tour guide. He can customize the tour for you - if you let him know you're interested in lighthouses, he can probably come up with the right tour for you.