At least you'll have decent weather, more or less. We had a b-i-g windstorm /rainstorm in the Cotentin Saturday night as low pressure broke the back of last week's high-pressure heat wave. Weather was clearing today and tomorrow is supposed to be partly cloudy with a high of around 22°C (it'll be cooler on the beaches -- probably 18° to 20°).
As far as advice, I guess I'd suggest spending some time this evening listing the things you're interested in seeing, pull out a map to see a logical progression, and plan to visit sights in a sequence that makes sense. If you're interested, there are two American cemeteries in Normandy: one (the larger and more famous one that you've probably seen in movies) in Colleville-sur-Mer near Omaha Beach, and the other, smaller but still impressive one (confusingly named the Brittany-American Cemetary although it's located in Normandy) in Saint-James, east of Pontorson and southeast of Mont St. Michel.