I live in southern NH and go to the Cape often (and actually lived outside of Denver long ago). Mid to late Sept is a great time on the Cape. The crowds start to thin out after Labor Day. What are you interested in doing on the Cape? I would suggest the mid to upper section on the Cape. The Cape Cod National Sea Shore is part of the National Park System, and is a great place to visit. But the beaches on the southern side of the Cape, on Nanutcket Sound have the warmest waters (South Yarmouth, Dennisport, Harwich). There are many great Irish Pubs on the Cape (The Olde Inn on Rt 28 in West Dennis is my favorite) You might also want to visit P-Town, or take one of the ferries to Nantucket or Marthas Vinyard.