There are so many great locations to watch the sunset across Prince Edward Island! My personal favourite is Prince Edward Island National Park - Greenwich. Here you can walk the boardwalk amongst the spectacular dune system to the beach!
There are also some perfect locations along the Prince Edward Island National Park - Cavendish, especially the ocean view look-out.
Really anywhere is perfect! You could enjoy the sunset along the waterfront in Charlottetown, or sitting in the comfy red chairs at Skmaqn–Port-la-Joye–Fort Amherst National Historic Site; the views will not disappoint!