One of the most peaceful ways to see the island is from the water; Brant Point Marine (32 Washington St., 508/228-6244, www.brantpointmarine.com [1]), rents kayaks, canoes, and other boats as well as fishing gear, trailers, and anything else you need to get on the water.
If you’d rather let someone else do all the legwork for you, find a spot on The Christina (Straight Wharf, Slip 1019, 508/325-4000, $25 per person), a mahogany catboat that sails regularly out of Nantucket Harbor. (The longest of the cruises, the 90-minute sunset cruise, is the best bang for your buck. For added romantic value, bring a bottle of champagne for the ride.)
Links:
[1] http://www.brantpointmarine.com