JetBlue and Airtran fly into Boston's Logan airport (BOS) and have very reasonable fares. Southwest flies into Providence's TF Green airport (PVD) and has discount fares. We have been told by some Cape natives that they find it easier to navigate their way from Providence and there can be slightly less traffic on this route, so that may sway your decision.
Cape Cod is equal driving distance from Boston and Providence, about 75 miles. There are only two bridges from which you can access the Cape; therefore, vacation traffic on the weekends can be heavy. Please allow for extra commuting time if you are arriving Friday evening or departing Sunday afternoon.
Boston's Logan Airport
http://www.massport.com/logan/default.aspx
Providence's TF Green Airport
http://pvdairport.com/
